What do I do?

My boyfriend and I have been together close to six years. We recently moved in together, we've been planning our future together we've looked at engagement rings have been looking for a home to buy and in the midst of all this he felt that he needed to be upfront with me and tell me that in the first 6 months of our relationship he made out with not one but two girls but he's regretted it ever since and it's been tearing him apart a lot lately as we've been making our future more concrete and I just don't know how to process this. What would you all do in this situation?