Diet for old sickly cat

I've been caring for a stray tomcat for a long time since it was little. It would visit almost on a daily basis and sleep a lot but then it started taking prolonged leaves of absence until eventually it got so claustrophobic that it resumed to random encounters when I would take it in my arms (once it would follow me enthusiastically but no protest) and take it home but usually it would head to the door after eating. Lately it is a frequent guest again but it has become thin and weak and it has a poor appetite besides drinking a lot of milk. I know what to feed it to make sure it eats but I'm concerned whether elevated cholesterol, triglycerides and stuff could only worsen its frail health. I'm reluctant to take it to a veterinary for now as I don't want to frighten it into avoiding me.

When an oldster loses his appetite, the thing to do is to just get them to eat,  not worry about the diet. Cats process cholesterol, etc. differently than we do BTW. Get him to eat what ever you can, tuna, hot dogs, canned chicken, etc, my old one absolutely loves Arby's so that is what I give him when he stops eating.  The most nutritious would be Gerbers baby food, chicken, turkey or beef, cats seem to love that too.

There is no way I could take my 9 year old feral in that I have had since a kitten, when he is wounded or 'off', I call the vet and say I have a wounded feral I can't bring in and I need an antibiotic which he gives me as over the counter. 

Cows milk is digested differently by cats, it usually causes digestive upset and diarrhea. Goats milk is fine, as is kitten supplement milk that is full of calories and nutrition.  I found some cat supplement milk made for oldsters on Amazon that was cheap and full of nutrition (it was an 'add on') and my old boy loves it, I freeze the left overs in individual servings., you might want to try that if he likes milk. All the luck, I hope he gets better!

Thank you, I'll try this approach and see how it goes.

Older ones need to be taken care differently from that of younger ones.  cats above 12 have arthritis which comes with pain and immobility. Dental disease can pose a problem. This can even lead to weight lose. Some suffer from kidney disease, thyroid, liver and heart diseases. Some become inactive with age

I would ask the vet for a senior blood panel and check of his thyroid and make sure he doesn't have diabetes.  As kitties age, they need a lot more protein in their diet, too.  You might try to feed him a grain-free single protein food such as chicken or turkey.  If not that, then maybe a high quality kitten food.