Need help with getting cat to GAIN weight!

I have a 2.5 year old orange male tabby cat named Marley. He is a big hairy cat. He is VERY long and lean. He has always been on the slim side but over the past 4months or so he has lost about a pound or so, maybe a little more.
This is right around the time I found my new kitten and brought him to in to live with us. His name is Miles. At the time Marley was eating about one half to one third of wet wellness food a day and he had free access to Innova dry food. Well the Innova didn't agree with Miles so I had to switch to something less rich. I ended up getting Nutra Source. He is doing much better on this food but I figured out about 2-3 weeks ago that I never hear or see Marley eating it!!! I felt horrible so I went and got some Innova/Evo to give to him. I keep it up on the cat food counter so he can eat it and Miles doesnt pay much mind to it....until a few day ago! So anyway now he is getting about the same amount of wet food and he has the dry food he likes. He seems sooooo thin. I can easily feel his spine and ribs. He ALWAYS wants treats! This makes me think he is hungry. He is eating his dry food again, but Im not sure how much. He was really sick about a month ago. He had a really bad acne outbreak and had to have meds for it. You could tell he just didn't feel well. BTW, he weighs 9.5lbs. I think the biggest he ever was, was 11.5 and that seemed to be a good weight for him. He could easily be 13-14 and not be overweight.
Any advice here would be greatly appreciated!!!

Talk to your vet. Do they like wet food? An extra wet meal that they both like can help as can a tad more dry per meal for Marley.

Are you 100% certain that there are no worms? Often when a new cat is brought in, and even if it was supposed to be worm free, they bring worms with them. I suggest you ask the vet about that and either take a stool sample in, one of the cats, or if the vet suggests it - just deworm. (alternately, a couple months of Revolution would work unless tapeworms are suspected)

I had Miles wormed 2x when I got him and both cats have had multiple doses of Revolution since we have had him. I guess I can't bee 100% positive, I've never seen any in the litter box.

Have you ever tried Nutri-Cal?

I've started giving that to Lears to see if he fatens up some (we are planning on eating him for Christmas)

**I hope you all know I'm kidding about eating my cat**

Well, you're probably good as far as roundworms go. Tapeworms can be harder to spot in the actual stools, but would periodically causetiny segments (that look about like a sesame seed) to be dropped around the house. Especially on their favorite sleeping spots. Always be on the look out for that (advice to all pet owners).

Again, talk to your vet. That's what they're for. With help, and vet approval, you can work to get his weight up. Plus your vet can check to see if there's anything more serious contributing to this weight loss.

Have your cat checked for worms if your cat isn't gaining weight, doesn't have an appetite and is lethargic? These could all be signs that your cat is dealing with worms. If your cat does have worms, he will need a prescription from your vet to get rid of them.