Sexualized play is not uncommon amongst young children. In cases like this, it is an exploratory behavior rather than actual sexual abuse from one child of the other.
You may speak to your boyfriend that children go through different development stages and their behaviors are congruent with what is expected at each age (even interest/curiosity in other's private parts).
The difference b/w perpetration and sexualized play is that in the latter, there is no coercing, dominance or threats being made from one child towards the other. It is a "mutual" endeavor.
When you talk to your son, help him understand that private parts are not something to touch in others or expose himself (even if he's curious or it feels good). You have most likely already spoken to him about respecting his own body and that of others. Most importantly, ensure that he is not shamed as you let him know that you understand that he had played this way but you'd like him to halt doing so.
Your Body Belongs to You
Good Touch Bad Touch: Learning About Proper and Improper Touches
Feel free to reply back.
"when will he forgive him?"
Your boyfriend is a grown up man and your son is a young child. If your boyfriend does not want to discuss it right now because he is still angry, give him some time. As far as when exactly he would forgive him, that would depend on his coping skills and ability to rationally look at the situation. He would also have to understand that neither your son nor his own son are error proof or curiosity proof. He can spend some time observing his own son to see whether or not he's acting out in an uncharacteristic ways or if he's carrying on as usual.
You could propose to him that the two of you speak to the boys together without expressing anger or shaming; go over the good touch/bad touch information and answer the questions they may have for the two of you as grown ups. Children need to know the reason (why) behind something that is being asked of them.