I think Ellis is right: apologize and address the issue. When I goof up with my husband, I apoligize and try to explain my feelings in a way that doesn't make him out to be a villian.
I've had to have a similar coversation,(as have many, many mothers) and explaining why it upsets me helps. "I feel that, when I am the only one consistently disciplining our child, it puts me into the position of being the heavy, and you get to be the "Fun One". I would like to get to be the Fun One too, sometimes. It puts our relationship out of balance when you decide not to follow through on correcting our son when he needs it, and it puts your relationship with him out of balance as well. He needs to see both of us as equally strong and caring and wants us to show him the right way to do things, not ignore him when he makes these mistakes."
There's something to be said in regard to our desire for our kids to just get along, or for us to feel they *like* us. I think some parents don't want to come down too hard, like their own parents might have, and they don't have any other way in mind other than the path of least resistance. Parenting classes can help, and taking one together with him might help too. Look around in your area; you might find one that suits both your personalities and values as parents. Get a babysitter and make it a date. This could be an opportunity to come together in the best interest of your son and strengthen your marriage at the same time.
Best wishes,
H.