Senior Coonie Boy recovering from starvation- Advice Needed!

[MandyCoonie]

This is Merlin. He was found by the dumpster at a local no-kill shelter, where they took him in and then straight to the vet. He was diagnosed with starvation, dehydration, extreme flea infestation and six hideously rotted teeth that had to be extracted. He only has two canines left now, one on top and one on the bottom. I think they took a molar as well. The vet estimated that Merlin could be anywhere between 12-15 years old. We had been thinking about adopting an older cat because we know that outcomes for senior kitties in shelters are never good, so when we saw his picture on their website we adopted him that day. He can only eat wet food right now, and seems to have a strong appetite despite having what I'm sure is a sore mouth! He tried to eat our other cats' dry food but I put a stop to that because he was just swallowing it whole- not good. He is the usual chirpy, talkative, follow-you-everywhere, head-butting Coonie despite all he's been through and gets along beautifully with our other cats. My other Coon, Mandy, is less than thrilled but is curious to see another one of her "own kind", and Merlin seems to be equally curious about her.
In addition to his wet food, which he is given three times a day, I've been researching what other things Merlin needs to gain weight and strength. We do not believe he is any older than 10-12, given his energy and playfulness--still an old cat but not the ancient cat he must have looked like when he was found. Incredibly, his kidney and liver function tests came back perfectly normal and his eyes are very clear and not cloudy. He's a complete stick figure right now, to be honest I've never seen any cat this thin but it's really sad to see what should be a huge, robust Maine Coon in the bag-of-bones state Merlin is. He's a big boy with bushy feathers or "snowshoes" on his feet, super long whiskers and a never-ending tail, and I look forward to the rest of his beautiful fur growing back in So again, we're totally new to having a cat with special dietary needs. Any advice or help is greatly appreciated!

-Karen