M.R.
We have this trouble too. Our oldest 3 are his/hers with only one of them living with us full time. With the boys it has been best if my husband disiplines them. He knows how they think and I'm a real softy which just doesn't work because they go too far. I agreed to back up my husband's ideas for a month. It worked so well that we do it all the time now. Sometimes I have to walk out of the room because I melt under the puppy dog face but my hubby's immune. Our kids are so much better behaved when they know they can't play us against each other and I've been able to step back in for minor infractions. If the disipline isn't abusive I'd back off and let your husband take over for awhile. If it helps discuss a code word between the both of you for when you think it's getting a little rough and agree that if the code word is used you will excuse yourselves from the room, cool off, discuss it and then go back as a united front. It won't take long before you son gets the idea