I am sorry your baby is not feeling well.
In a cat his age, my first suspicion in abscessed teeth in the gum giving him pain. The next is acid reflux. Then there is hyperthyroid, dehydration, renal and liver disorders.
There are a few possibilities for these symptoms. I will try to cover everything.
Weight and hair loss is usually related to hyperthyroid. The cats eats great but the excess thyroid hormone burns up all the calories.
The problem is that this disorder can lead to your baby developing diabetes, high blood pressure and other ailments.
We do have excellent meds for this but is is necessary to do a T4 thyroid blood test.Any other thyroid test can be inaccurate.
It can also be related to renal issues especially if there is excess shedding because the hair becomes very dry.
Stress weakens the immune system and can lead to illness.Dental disease also depresses immunity and sends bacteria into the organs.
If he eats dry food that may be the culprit. PLEASE READ:
Lack of fluid in the bowel also leads to vomiting, Ingested hair can also be a factor.Anorexia for any extended time leads to liver damage and causes nausea and vomiting as well.
I would add some canned or baby food for him
Serve the two foods separately - do not mix them - add a half pepsid a/c tablet, crushed up into the wet food one time a day.
human baby food in meat flavours (check that there are no onions or garlic in the ingredients) and mix that with warm water and offer that, or syringe it in little bits into your cat's mouth. Beech Nut makes a line of baby food that has nothing but meat (beef, chicken, turkey or veal) in it. Here's a link: http://www.beechnut.com/Our%20Baby%20Food/Results.asp If you cannot find this, you could find another meat baby food - just read the label carefully to be sure there are no onions, onion powder, garlic, or garlic powder in it.
Cats are attracted to smelly foods - the stronger the better. If she wants tuna, give it to him You can also try fancy feast, clam juice or any canned.
He should have a check-up in the next few days.
Please let me know how he is doing.
Best wishes for him,