It's not a surprise, @Waffletots .
What's ahead of him in terms of inconvenience and unpleasantness has just dawned on him.
He's not even pretending it's about your relationship.
I'm sorry if that seems really blunt but I'd think very carefully about that message. It's manipulative and a 'please don't make my life hard' request.
You are young and it sounds like your parents are also young enough to be helpful practically and emotionally. Your brother and sister are in a place where they can support you. You have maximum support now - if you went back to him and waited five to ten years - what would your support network look like then? This would be a big consideration for me.
Could you ever view him in the same way as before the OW's reach out?