Please help this new cat owner!!

So here's a little backstory:

I adopted Marlon a little over a month ago from my local county shelter. He's a 10 year old tabby who previous owner was taken into a nursing and was not allowed to take him with her. He is declawed in all four paws. He had spent the first month at the shelter and they told me he had diarrhea and was vomiting due to his change in circumstances (stress). He, then, spent a month in a foster home where he also had diarrhea and vomiting for th first few days but then he got better (I'm assuming as he got more comfortable). We finally adopted him after he had silent about 3 months in between the shelter and foster home. The shelter had given us a clean bill of health and we took him to the vet a week later for a general check up. The vet told he was fine but he did need to lose weight (he weighed a little over 17lbs). He's not an active cat. He doesn't chase, play, or do much of anything else. I've tried and bought him toys that were listed as his favorite but nothing. Occasionally he'll swat at a string but not much. She just suggested I give him smaller portions when feeding him. Since I've gotten him, I've been feeding him Purina Cat Chow for Indoor Cats (he's strictly an indoor cat since he's been declawed), a third of a cup in the morning and a third of a cup in the even following doctor's orders. I know it's not the best but I got 2 full bags of it from the shelter for free. He's had no problems at all especially in terms of digestive or gatsrointestinal problems. He hasn't even had stress diarrhea or vomiting like he did at the shelter or foster home. I was really happy because that meant he was really adjusting well.

So fast forward to Monday:

I finally decided to give Marlon wet food for the first time since getting him. I wanted to make sure he was settled and dry food was working for him before I changed anything. I gave him Purina Veterinary Diet EN Gastroencentric Feline Formula canned food. He ate it just fine for dinner and a bit throughout the night. However the next morning (Tuesday) he threw up twice. I gave him his regular dry food for breakfast and decided the canned food I had given him wasn't for him. He had diarrhea for the next two days (Tuesday and Wednesday) but no more vomiting. Wednesday I had looked up ways to stop diarrhea and I had read that a 24 hour fast is good to clear out their stomach. After I saw he was still having diarrhea Thursday morning, I decided not to give him any food for the day. He had been eating and drinking just fine in the 2 days he'd been vomiting and had diarrhea. I made sure he had fresh clean water on Thursday. Other than having diarrhea Thursday morning, he didn't have any more the rest of the day. I gave him his regular food Friday morning and he ate about half of it and promptly threw it up. But he ate the rest of it and didn't throw it up. He's been eating and drinking just fine as of yesterday (Friday) and today with no more vomiting. However I've noticed he hasn't used him litter box other than to pee a little bit this morning. Before he used to pee about 3-4 times a day and poop 1-2 times a day.

Other than the stomach problems, he's still his old self. Sleeps most of the day, eats and drinks like he did before, and "plays" like he used to.

So I'm wondering, is this something I should be worried about? Should I just wait to see if he eventually poops and goes back to his normal digestive routine? If so, how long? Is there anything I can give him to help him poop if he doesn't eventually go? And lastly, is there a relatively cheap brand of canned food that you recommend for a cat with an apparently sensitive stomach?

Sorry this was so long! I just wanted to be thorough!

if the cat wasn't throwing up or having diarrhea with purina cat chow for indoor cats, why not stick with that?  he can become dehydrated from all that diarrhea and vomiting, so yes - you should be concerned, particularly since he's stopped peeing.  also did the vet take a stool sample and blood sample and look for common diarrhea producing issues? 

@basschick
The cat food I was giving him was dry food and I wanted to mix in some wet food into his diet; that's why I gave it to him. Also, he's been eating and drinking just fine especially after his diarrhea and vomiting had stopped 2 days ago. He's not sluggish or slow. In fact he's running around right now as I type this (kind of interesting actually since he doesn't run much ever because he's been declawed). Do you have any suggestions as to what I should do as of now?

And yes a stool sample and blood tests were done when I took him in but he didn't have any diarrhea or vomiting at that time. Everything came back fine (as I assumed it would since the shelter told me he was happy and healthy).

It sounds like the canned food did not agree with him. However, I do agree that you should incorporate some canned food in his diet.  You might try something that's grain-free and a single protein source like chicken or turkey.  I would avoid anything with fish as that is often difficult for some kitties to digest and can cause stomach upset, vomiting, and diarrhea.  Most likely, his weight is the cause of his inactivity, not being declawed.  I think once you get a handle on his gastro problems, you can work on his weight. 

Also, if you need to form his stool up maybe you can incorporate some pumpkin in his diet for a bit. Mix it 50/50 with his wet food if he won't eat it straight. My cats love it. But I was too wondering if maybe he picked up a little something. I would have a vet do a fecal on him. How sweet that you adopted an older cat!

I am happy to say Marlon has finally pooped! LOL it wasn't much but it was something so I'll still keep an eye out!

@stephanietx

Thank you for your advice! I will definitely look into more grain-free options for him. I was told at the shelter that because he was declawed, he has difficulty putting pressure on his paws so assumed that's why he doesn't run around much. I don't know much about how he lived with his previous owner but seems like Marlon wasn't played with much or he just seems very disinterested in doing so. Regardless, he purrs, meows, and rubs up against me when he sees me, and cuddles with me during bed time so I'm just happy he's happy! He's lost some weight in the last few weeks since our vet visit, so if it his weight that's causing him be inactive, hopefully soon enough he'll be running up a storm!

@EastwoodNJosey

Thank you! I will definitely keep that in mind if something like this happens again! I think putting him on that 24 hour fast threw off his digestive routine. I will keep an out eye and if it doesn't get back to completely normal by week's end, I'll take him in. He's eating, drinking, and urinating perfectly fine as of yesterday and he finally pooped today so hopefully it's nothing at all. As for adopting an older cat, he's got a face you can't walk away from! I fell in love instantly and adopted him right then and there!

Good luck and keep us posted!