I am sorry your boy is not feeling well.
Since he goes out it is possible that he ate something spoiled or even toxic.
Check inside his mouth for any abnormalities.
An anal discharge can be from colitis, parasites or his anal sacs:
Offer him the juice from a can of tuna or salmon to keep him hydrated till he can be seen.
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.
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,