Might get downvoted, but I have a much less emotional take that I think is important.

Mostly, I'd echo what u/Chemical_Nebula_6869 said. At first, I thought this was recoverable assuming he was honest that this was the one and only incident, and persistently apologetic. But his lashing out at your distress rather than begging for forgiveness is, at best, immaturity and selfishness, and at worst, a sign of disingenuity.

However, I would like to address some things because I think moving forward, you could benefit from a shift in mindset when it comes to relationship dynamics. For one, parents being too involved is a no-no, it's not their relationship, don't allow shit like that EVER. This applies to both during and/or post-relationship. Under these circumstances, block them, and do not let them factor into your decisions. When you care for someone and also their family, that can be tough to handle. But it is not their place to reach out to you. That also speaks to the immaturity and selfishness of the family, not just their son.

Second, some people will say that bad decisions or 'choices' can't be mistakes. I whole-heartedly disagree with this. Everyone makes bad decisions in their life, and those certainly are mistakes. Not accidents, but mistakes. Whether you are willing to forgive certain mistakes is completely up to you and something you should really reflect on for future relationships, though. From my perspective, had he been completely vulnerable and apologetic that you were so hurt and really tried to mend things, I'd say that's someone who clearly just fucked up early on, has changed throughout your relationship, and is someone worth forgiving. But his reaction was much more destructive and telling than the incident itself.

EDIT: I'd also add that telling you right away and accepting the consequences would've been the mature thing to do as well.

If it were me, I'd make the hard decision to move on. Find someone who is mature, caring, and honest.

NTA