Time to talk. You understandably are still hurt and have some unresolved feelings about what he did. He doesn’t like to talk about it? Too bad. Boo-hoo. He shouldn’t have cheated.
If he wants to be with you, he should be doing everything in his power to prove that he is sorry and can be trusted. He should be willing to talk about what happened with you whenever you need to. Plus, he should be paying attention to your needs and realize that isn’t something that you’ll just get over. It could take a lot of time before you completely forgive him. Also, you might not ever fully be able to forgive him. I’m glad your relationship has improved, but he still needs to put in the work.
The fact that you were both each other’s first sexual partner is irrelevant to what he did. If he was feeling like he wanted to sleep with someone else, he should have broken up with you or suggested a break. Instead, he cheated, and now he has to deal with the outcome. There are still issues that need sorting out if you’re going to be able to fully move on. He can’t turn off a switch and assume everything is fixed. There is still a lot to repair in the relationship. The more he refuses to talk about what happened, the more resentful and unhappy you will get.
If he’s not willing to talk and put in the time to really 100% fix things, you should consider whether he’s really the person for you. While many couples do survive infidelity, it’s impossible if one half refuses to discuss what happened. If he is willing to talk about it, and you still can’t move on, you should also consider whether what he did caused irreparable damage to the relationship. But it sounds like all he needs to do is to stop ignoring his mistakes and deal with the consequences of his actions.