You have also posted a similar version on the child behavior forum
Well, he broke up with me anyway, so he won't be around her any longer. I checked around with his daycares and others who have had him around their kids, and he has never hurt a baby or done anything inappropriate to any children. He even often helps take care of the babies when he is there. I do spend a lot of time with him, and he was a little jealous, but he said he did it because the last time she threw the bottle, it hurt, so he didn't want to hurt her but didn't want her to be able to see him to throw it again. My son just turned ten and the baby was 10 months. He doesn't really have any anger issues and is normally very caring and always helping people, so it surprised me that he would even hold her head between his legs. He had no access to undoing his pants though, and had both of his hands on her head, so I don't think it was sexual. The more I analyze, the more I think he just handled her throwing the bottle wrong.
I agree this is a concern and why would she throw a bottle at him , how old is this child ?Is the childs mother in the picture, how often is your son with her.Supervision is definatly needed ,and maybe if he is angry some anger management .
I wouldn't leave them together alone, frankly. Either your son is experimenting with no concern for her safety, which is a bad sign, or he is angry, which is a bad sign also. Are you spending a lot of time with the boyfriend and he is used to you spending more time with him?