dipping baby under water

My boyfriend was giving our 3 month old a bath and his grandma walks in and tells him to try and let go of his head and see if he can float. I already knew he couldn't so my boyfriend went ahead and try it and our son flung back as soon as he let go and went under for a few seconds. I got so mad that he did this. The look and the way this happened scared the crap out of me and I wanted to smack my boyfriend for it. Our sons mouth opened as soon as he went under. My boyfriend thinks I'm over reacting for being mad and upset and tried to reassure me it was fine. Do you think I'm over reacting?