'High-Five'
And what I mean by that is that every time he becomes fixated go give him a 'High-Five' and have someone temporarily move the girl, or when giving the 'High-Five' have an additional distraction (like snacks) ready, and after a while of that, he'll break the urge.

I remembered my daughter did the same at the daycare. I tried redirecting her, visual scheduling to let her know that it was not a good idea. Finally after a lot of repetition, she stopped that behaviour.

Try getting him fidgets. Could just be a sensory need he is seeking

This sounds like my oldest son who does various things to see reactions. He used to pull his sister's hair and poke her in the eyes. Thankfully he got out of that phase. Maybe try redirecting him with a special fidget or toy. It's so hard to find out what works. It seems to take A LOT of repetition.

My nonverbal son used to do that but grew out of it. He also liked to play with string then. We would redirect to the string but it is a difficult behavior.