He is a nasty human being but if you are not a romantic couple, he is free to post whatever he wants on IG and do whatever he wants including dating or going to parties. I don’t understand confronting him about some girl. As much as it might hurt there’s nothing you can do about what exes do when we leave them. I honestly think it’s better to stop following him or contacting him. I do understand you are hurting but confronting him about what he does or doesn’t do just doesn’t serve any purpose. You can do it. You deserve better