[X] Ask him straight-up what's wrong. This is about as terrifying a prospect to imagine from his perspective and yours, but if there's a chance it might be the key to all this you can't dismiss it.
[X] Go for gold. Try to convince him that you know how he feels and he can trust you to talk about it. Share something... personal. Fuck you hope it doesn't come to that. [x0.75]
[X] Sidetrack the conversation by talking about random shit. Belial's mood may have a chance to improve if you pretend like you haven't noticed it, or at the very least he'll have to acknowledge you're still trying.
[X] Try to sort of sidle up on what's wrong from an oblique angle. Hedge your bets some, give him an out to talk about it without really talking about it.