confusedinTX

First let me say good luck. I have issues with my father in law and am "lucky" enough to have our bro in law living with us. There is no easy answers to this one because it sounds like your husband is in the same place mine is in that he lets his parents take the lead and doesn't defend you. It is hard to be in the middle but I have to think at some point it should be about what is best for his family and his son. A lot of kids need a schedule and that alone could be why your son is acting strange when dropped off.
We fight a lot about his bro in law who is loud, messy and overall disrespectful. I have made the case that it is time for him to move out but have had the best results in framing it in that it is in the bro is laws best interest. The other way I have tried is to talk only about what is best for our daughter. Either way he seems to respond better then if I bring up anything involving myself. Maybe you can find a way to approach it where you don't bring up your feelings on the matter (especially about his parents). If you don't think you can come to an agreement with your husband then you might have to make one and risk him being mad at you. A tough choice between doing what is best for the kid and pissing off the husband but sometimes it has to be made.
I think a lot of inlaws second guess our parenting styles and if that is just the way they are that isn't going to change. I try to just live with it and try not to let it bother me so much. I know that is harder when it doesn't feel like your husband is on your side. I could tell you the horror stories from my inlaws but really what it amounts to sometimes is they probably wouldn't be better with someone else so I try not to take it personally. Good luck.