What if he is seeing someone else? Bummer! But don’t let that kill everything. Unless they are married or engaged, there is still hope. Tell him he can invite her, too, and make sure you are cordial to her. You don’t have to befriend her but don’t treat her badly. Be polite. Let him know that you are happy for him if he is happy. And then just wait and see how it all comes out. If she is the one for him, they will stay together. If he is still feeling you, he will eventually let you know. He could be seeing her (and others) because he is trying to get over you. Or because he didn’t think he had a chance with you. For now just resume the friendship by touching base with him once a week. If he wants more contact, again, he will initiate on his own.
Patience might be key in this situation, so just keep dating other guys and continue the friendship. If it is meant to be, things will work out.
If you’d like more specific advice, contact me or another dating coach.