Was seeing a guy for 4 months quite regularly and had met all his friends quite a lot.. I was pretty involved in his life. I ended it due to bad timing (he's going away for a month on his own to party then come back and settle down) and something else that pissed me off but in the end had a discussion and agreed mutually to go our separate ways but I still care about him and want to get back with him when he's back.
Straight away he started posting stuff that is inviting for girls and just now a girl posted a picture of him in her bed asleep. I deleted him straight away because I just don't need to see that.
First of all, very understandable that you deleted him. In general I can tell you that men react better to words. Otherwise the guy will just not know what is going on. Communicate what feel by using real words and not hnts. So, I vote for telling him.
By the way you don't have to delete him. You can just "unfollow" him. You will not see any updates of him while you still remain friends.
Since you both accepted to take your separate ways, what is the point of you still caring about him? As harsh as it sounds if you would really care about him that much you would have not mutually came to an agreement to break the communication.
He understood it is not possible anymore and to be clear he is a douchebag so I doubt it will make it better or worse if you will explain to him why since he started inviting girls. You basically fell for someone that is not into you as much as you are into that person.
Leave it. You made a gut reaction move. So unless he asks, then don't bother. If you're going to repair the situation, then feel free to explain you were bothered. But if you're just wanting to express your feelings, spare him.