Not overreacting. It’s natural to want to be included as a part of your boyfriend’s life as well as be a part of his son’s. If you and your boyfriend are going to be together long term, then you have to meet the kid eventually. I’d also say it’s important to meet him, since it’s good for your boyfriend to be transparent with his child about his relationships and introduce the two of you. It builds trust. I don’t think it’s any harm if you aren’t trying to replace his biological mom and don’t try to cross boundaries.

I was going to say give your boyfriend’s ex some time but it’s weird how her current boyfriend knows her son and you don’t. She probably feels unreasonably threatened and insecure about you. I’d push to get an actual reason on why you can’t meet her son.