I think you need to talk to BF, without the drama.
In a blended family, you can't have everything. I know that hurts, but the bonus is that your son can have it all.
There were a couple semi-sane holiday/birthdays with BM when she was the hero gift giver and the boys liked her present better than the one from DH and me. It stung, especially when it was what I told her they would like, but the important thing was that the kids were happy.
There was also another Christmas when the boys were very young, DH did all the wrapping and labeled every last present from "Santa" so the boys didn't realize we had gotten them anything. I thought he was nuts, but DH didn't care. He just wanted them to have the presents.
Hopefully, you and BF can find some compromise. Will BF/gf be there Christmas morning? Can they give it to him then after the Santa-brought helmet? He can be excited to have two?
It sounds great for LO, though, since he can have a helmet at each home.