I've been in the same situation and I know the answer isn't what you want to hear. If they are both happy together there isn't much you can do. But you should definitely tell him. Maybe he feels the same way but doesn't think you like him. And if he doesn't feel the same way then you need to move on. Good luck!
I think this is a situation for you and your friend to solve, not strangers on ask. Wether that means telling him how you feel, getting over him, Or maybe breaking a friendship. Do what you feel is right.