I would post it either in health or in general.. and do a search for 'vegan cats', there are other threads on here about it, it was even mentioned recently.
You didn't mention in what way he is sick (as in, is it related to what he's eating) or what you're feeding him now.. I'm assuming you know that if you're feeding him a homemade vegan diet then it absolutely has to be supplemented with Vegecat powder right? On the manufacturer's website http://www.vegepet.com/forcats.html
there is a link to the literature page, and the recipe/instruction book does have recipes for TVP-based foods.
Have you taken him to the vet yet? Vegan male cats in particular might get urinary tract infections.. (actually in that other recent post I was mentioning how the Farm Sanctuary and Evolution cat food people are working on a vegan food appropriate for those cats) if the problem is something related to his diet you may have to switch back to regular food, at least temporarily. I know it would suck to have to buy a meat-based food if you are very opposed to doing so but if he needs it right now for his health...
Let us know what his illness and food situation is and what the vet says and stuff! (or, well, post it in the general forum). I hope whatever it is isn't serious and he gets better soon!
Sorry if this post sounds weird or anything it's hard to know what to say, knowing so little about the situation, I'll reply better to your next post
Oh, and welcome to the boards!