Boyfriend wants to disciple my son

I got into a huge argument with my bf over this, my son is 5 years old and throws a tantrum every other day when it’s time for school, my bf hates this & has told me that he’s going to start disciplining him( spanking) I got very upset because I don’t even hit my child, my child has his father in his life & doesn’t need to be disciplined by another man. My bf is good to my child, he treats him as if he was his own but I just can’t tolerate spanking at least not at his age. He believes I am wrong for this because he does a lot for my child & believes he has a right to discipline him. Am I wrong?