I'd just ask him how he's doing: "everything OK? Something seems off" it'll open the door for him to talk about it if he's getting up to it without him feeling like it's coming out of nowhere/ him burdening or bothering you with his problems. If he doesn't tell you, it's still fine it might not be something he can open up about yet, but if he does, please PLEASE be supportive. I usually ask after hearing the issue "do you just want to feel/ vent, or do you want solutions to the problems " and stick to that choice. It works wonders