I am sorry your cat is ill. The pediasure won't hurt him but could give him diarrhea. He may have a urinary tract infection so he should be seen today.
PS: Do not give him any more aspirin!!
UTI's are serious in males because they are caused by sticky crystals in the bladder.At any time they can block the urethra.Since this is going on a while your boy is in great danger.Cats are very stoic animals so he may not cry out.
If he becomes unable to urinate that is a MEDICAL EMERGENCY . His bladder could burst and he would have to be catheterized to save his life.
UTI's must be treated with an effective broad spectrum antibiotic like Clavamox or clidimycin and for a long enough duration so all bacteria is eradicated. If not, they will refluorish, stronger and more resistant and reinfect him. In addition, the bacteria can become tolerant of the same antibiotic so the medication should be varied for best results.
You can give him a few drops of unsweetened cranberry juice or 100 mg of vitamin C - That will reduce the PH of his urine but it is only to make him feel better.He still needs to see a vet for an antibiotic.
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 immedately
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,