I would try to find a way to let him know that if he ever needs someone to talk to that you are there for him, that he can count on you to not share what he tells you, that you won't judge him, etc. I wouldn't talk to him about what you experienced or ask outright if anything is happening as that could potentially backfire and create trouble where there is none.

Hopefully by opening the door and building that sense of trust with him he will come to you if there's something bad happening.