I would play the "I'd love to, Darling, but I'm really broke at the moment, and I have to work" card. Do that until he suggests he start paying again. If he doesn't, at least it was his decision, and he won't feel so pissed off. Alternatively try the, OMG, I'm soooo busy, I'd love to, but I just can't seem to find the time. When up against it, the "my period is late", or the "I wonder what that weird green discharge means" strategy?
Doesn't really matter, so long as you engineer him ditching you, not you rejecting him and him getting pissy. The fact that you fear a bad review means you already know deep down that he's NOT a friend, and the problem goes beyond not getting paid. You don't trust him, AND you're wasting your time. Since you seem to already worry a bit about aspects of his personality, how long is the sex going to feel good? I know great sex is addictive - been there.
All else fails, ring him up at 2 in the morning, in a fake panic, and tell him your violent ex has just been discharged from jail, and is pacing up and down in front of the house, looking furious. Would he mind coming over and sorting him out i.e. tell him that he's your boyfriend and protector now. Bet you'll never hear from him again.
What a great opportunity for honing your acting skills.