Switch gone wrong?!

Please provide any thoughts or info you may have to share, I am at a loss!

Poos is an apx 9 yr old male, neutered cat. I adopted him 7 years ago. He has been fed a generic, store brand dry cat food for all 7 years. At times he would regurgitate (he would eat too fast) and like other cats, there have been occasions of vomiting - nothing to worry about. Anytime I would take him to the vet, I would ask about this but the vet never seemed worried. Here comes the issue:

Approximately 3 weeks ago I switched Poos from Purina Indoor formula ((still feel guilty for feeding him this for so long ))  to Natural Balance LID Salmon and Peas. I made a gradual intro after a day of fasting (mixing the old and new didnt seem appropriate in this case) which went over fine. However about a week and a half in to the new food, he started vomiting. At first I chalked it up to the grass he had clearly consumed and deemed that he would no longer be allowed outside without direct supervision. Three days later I began to worry it wasn't just the grass. Poos proceeded to vomit about 30 minutes after eating... every time. I fasted him for a day, fed baby food for two days, and then fed him wet cat food for a day. Yesterday I mixed wet food / dry food (Canidae GF Pure Chicken) at about a 50/50 ratio. Everything went fine. This morning I fed him dry only and sure enough, 30 minutes later he threw up! 

I was convinced the Natural Balance was too rich for him... that was a pretty drastic change... but now that the Canidae is having the same effect... I am not sure what it could be. Everything wet stays down just fine. Anything dry, comes up a period of time after it has been partially digested (puke always contains food that has clearly begun breaking down). 

PLEASE any thoughts? - I am not opposed to an all wet diet and have now realized this is even better than a GF dry food. I want to switch him to all wet because I do not think he is getting enough water (he is INCREDIBLY picky - he will drink out of the toilet but not a brand new, clean glass cup that is full of bottled water) but I now have two bags of (what I think is) decent cat food. I'll throw away $35 if that is what this means but wanted to reach out to my fellow kitty parents before I do!

BTW -  At 15 lbs, the vet said Poos should not gain any more weight but was ok (he is a bigger sized cat). I have estimated that he has lost 4lbs over the last two months (partially due to this situation partially due to the smaller amount of Purina that I was feeding him right before this). He still looks very healthy and by no means is underweight but I really don't want him to get there. Also, his fur keeps improving. He has always been a handsome fella but he is SO shiny and soft now, and he sheds much much less. 

Please let me know any thoughts or ideas or if I can fill you in on anything I missed. 

Thank you so much!!!  

Initial thoughts...protein intolerance, possibly to fish and chicken. The Purina Indoor has very little animal protein in it compared to the other ingredients. So, the fact that he had periodic vomiting on the Purina could also be a link.

I might suggest trying a LID duck, rabbit, venison or lamb diet (canned is available in Natural Balance, Nature's Variety Instinct.) Just read the ingredient list to make sure there is no fish or chicken (or chicken fat.) Also, some cats with chicken allergies cannot have eggs either.

For me, I'd throw away the dried food or donate it to a kitty shelter. No amount of money is worth him having so much trouble! My cat pretty much always throws up on dried food as a main meal, so I only every offer him about 10 pieces now and then as a 'treat' as he does like the taste of some. My previous cats did the same too.

Can I say something about the grass? Grass is good for your cat - they do not only eat it to throw up. In actual fact, when they eat it to throw up it can be clearing out the system - hairballs or something. It also can help them digest hair (for exit from the other end) and contains folic acid which they need. Please don't stop him eating grass if he wants it. Cats tend to eat it when they know they need it and am a tiny bit concerned that your supervising him means you will not let him eat grass.

Catpak - Would a protein intolerance be evident when feeding wet food?

Fleabags mom- I never knew! I have been told the risks associated with outdoor grass outweigh the benefits... in this case it sounds like outside might be just what he needs. Ill provide an update tomorrow

Yes, intolerance to a particular protein would most likely be evident when feeding canned or dry, so depending on what's in the canned foods you've been feeding him, that's likely not the problem.

I don't know how the ingredient lists compare between the foods on which he throws up vs. the canned food he can keep down, but perhaps there's a shelter near you that would appreciate the donation of the open bags? I don't know that they take open bags, but at this point, I agree: it's not worth keeping the dry food.

In fact, the all wet diet is much healthier for him, for a number of reasons. The canned food isn't subject to the same high-heat processing the kibble is; the ingredients in it are subject to less processing, and canned foods *tend* to have a lower level of carbohydrates: most kibble uses fillers and carbs to make the kibble stick together. If you'd like to read up on the importance of moisture in the diet, this site is written by a vet, and makes feline nutrition very easy to understand: http://www.catinfo.org

If he's doing well on the canned, that's what I'd feed!

If it is a protein intolerance then, yes, it would be evident when the protein is fed in canned form.

He could also have an intolerance to the animal digest that is sprayed onto dry food.

If he can tolerate chicken or fish canned food, I would look at what is in the dry vs the canned.