I wouldn't press it too much.
If you keep asking to much he'll never want to tell you and will just shut you out.
Just make sure he knows you're there and that you're open to talk whenever he needs it.
Also just talking in general might bring it out.
Instead of going right out and asking what is wrong, ask him how his day went and make small talk.
He may just get the urge to ask advice or tell you what is going on if he knows you really care.