How do I handle this??

My oldest son has autism and my stepson (around the same age) keeps bullying him and just saying mean things. It’s to the point that I don’t even want to be around while it’s my boyfriend’s weekend to have his son. I’m starting to assume it’s the stepson’s mom that’s telling him to treat my son differently. We had a small altercation mainly because of her jealousy towards me because her son talks about me every time he’s with his mom. She felt “disrespected” I’ve never had any problems with my stepson disrespecting me or my son until that altercation. I’m conflicted.