How bad is the pet allergy? Would you be able to get by on antihistamines for a little while at your friends, as you search for a new place?
His family has abandoned him, and won't even answer my emails or calls. They have dumped him on me
When his own parents don't want him. That says alot about what kind of guy he is.
Not your problem. He needs money? He can get a job. He can't find a job? Plenty of empty cans out there for the struggling entrepreneur.
has been unemployed and living off of me for a long time.
I'd say it's time for him to pull up his big boy pants, find a job, or start practicing the sob story he'll be using to illicit sympathy from the passers by whose spare change he's begging for. Or write the apology for whatever he did to be disowned by his parents, so they let him back in.
I read the part about how you wouldn't feel right about just throwing him away, no problem. I have a plan.
Take him out for dinner again, order something you know he'll get pissy about. At some point get up and go to the "bathroom", then just leave. Stiff him with the bill. Then he can wash dishes to pay for the food he can't afford. If he does a good job the restaurant might hire him on, if not, then that's his own fault. Maybe then he will learn the meaning of the adage "don't bite the hand that feeds you.". At the very least you tried to find him a job.
It makes me so sad to hear you defending him even after all the shit he's putting you through, that tells me that you are a wonderfully caring, kind, loving person. You deserve so much better, you really, really do.