Despite the physical distance of online communication, I'd be concerned that the online flirting that he engages in could seriously undermine his relationship. Thus, intention and level of openness with the existing partner are determining factors in when someone crosses the boundary.

If someone has blocked you, I think it's made abundantly clear that they want nothing to do with you. Pursuing it further is only going to hurt you. Forget about it and try moving on. If the person wishes to talk you again, they will contact you again.

Ask how he feels about your relationship. You can ask him how he feels about your relationship so far to see if he still wants to be with you. If he does, he'll probably say that the relationship is going well. Say something like, “How do you feel about our relationship?” or, “Are you happy with our relationship?”

It is in effect ending the relationship but if he really cares about you he wouldn't keep playing this game with you. So you can just say, “Okay, You blocked me - I am going to make sure you don't get another chance to do that in the future. Bye. “