J.S.
It sounds like you and your husband may agree on how to discipline, but now how to carry it out. The best thing to do when the other parent starts to discipline for whatever reason is to sit back and let the other parent handle it. If back-up is needed, the parent sitting back needs to wait for the parent who's handling it to ask for help. As soon as you begin to handle a situation, if you're the first one to step in, let your husband know that you're handling it and if you need help/support/back-up then you'll ask for it. Same should go for him as well. If he's the one who initiated the discipline for a particular incident, you do the same.
It's GOOD that he wants to back you up. He's just jumping in too soon. A good time for him to pipe in would be when your son turns to Dad and says "But DAD! Mommy is being mean!" or you actually say "I need some help here."