Hi. Sorry,this is disappointing/sad for you. I think the only thing to do is nothing. If he gets back with her, just tail off your contact,outings with him. They may happen anyway if he has a girlfriend again. Don’t try to be friends,it will just be hurtful and painful for you.
I would not tell him your feelings. If he had some interest in you that way,he likely would have shown it by now,and he would not be rekindling an old romance. Start going on outings more with friends and think about meeting someone else to date. I wish you well.