I'm with Cassie on this one, if he's a boyfriend then no he shouldn't be disiplining your son but talking to you about what happened and leaving it to you to discipline him.
But if by boyfriend you mean he lives with you and takes responsibilty for your son as much as you, then you need to take this up with him and discuss how you will both discipline him in future.
If he has disiplined in an inappropriate way then he needs to be big enough to apologise to your son, not taking back that whatever your son did needed disiplining but the method that he used was ott and he regrets doing it. If he is unable to say sorry for it then I would seriously veiw this as how he does things rather than a heat of the moment thing.
If he continues to use that method you need to consider the effects on your son and maybe accept he's not the man you think he is.

Hey girly, he's only your boyfriend, what gives him the right to discipline your child? It's inappropriate and you need to tell him to stop...how old is your son? Just tell him that he doesn't need all that discipline that he's a good kid...Hope that helped :) :) Haha- I love doing that-Cayyla

Hi its me. He is a boyfriend of 2 years and is here quite a bit. If he is disciplining out of anger and yelling or just being unfair (according to me)do you think it is ok to interrupt and say," Excuse me, could we talk first?" I teach my kids to calm down first if they are angry, can I do that with my boyfriend with out making the kids think I am belittling his authority or him feel like I am belittling him. I don't know if that makes sense.