I am sorry your boy has this issue.
You may be seeing the third eyelid which appears in times of pain, stress or illness.
It may be from the flea treatment.As long as the product was for cats and not dogs he should be ok. If it was a dog med he should be seen in the ER immediately.
Otherwise just push food and fluids and he should be fine.
Cats are attracted to smelly foods - the stronger the better. If he wants tuna, give it to him.You can also try fancy feast, clam juice or baby food.
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.
If his tummy needs a rest, feed baby food with no onion or garlic for a few days.
If he wont eat, and your cat is not diabetic begin by giving him two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When he feels hydrated he will lick up the gruel on his own.
Give him droppers of unflavored pedialyte a few times a day to make sure he stays hydrated.
If he throws it up have him seen by a vet.
Please let me know how he is doing. I will always be here for you.
I hope he feels better very soon.
Sincerest best wishes,