You CAN help it. You need to get on the same page when NOT in the middle of a discipline episode. My husband and I agree totally on parenting, but I'm pretty much guiding him-like you, I'm the main disciplinarian, and he's barely home-so in THEORY, he's right on track, but when he gets home from a long absence, he's out of practice. For instance, he KNOWS we don't give lots of warnings, get exasperated, yell and THEN discipline, or ignore things, we just act after a warning, which keeps all the behavior on track. But when he gets home, the kids test him, and since he's out of practice, he accidentally ignores stuff, or starts giving too many warnings and getting mad (after he's ignored stuff, so the kids know they have all kinds of leeway...grrrr...). ONCE or TWICE in an "emergency" I've interfered. But 99% of the time, I let it go, and approach him afterwords in private. This way, he gets what I'm saying, and knows what to do next time, and doesn't feel offended. I make a POINT of not BEING offensive about it too. I phrase it as "You did really great in ____(whatever random distraction to get his guard down) but REMEMBER, if you give too many warnings, they get used to getting that many chances before they have to listen..." and he does do better the next time.
So. Get on the same page, but don't interfere WHEN the discipline is happening. Ask your husband if he wants his son to listen to him, or not. He may not care that his son is ignoring him. If he doesn't care, and he doesn't want to discipline it, you have to sort of accept that, so don't be accusatory, just find out. If he DOES want his son to listen to him, "support him" in making sure he keeps his authority as best you can.