A: Ah, the double-edged sword of "help from family members". Of course you want your rules to be consistently followed by your son's caregivers. On the other hand, you'll never be able to take away a grandparent's desire to spoil the grandchildren (and their belief that THEY know best). In other words: you can't win on this one.
Does it help to know that you're not alone? This is one of the most common parenting dilemmas out there. Only you and your husband can decide whether -- or how often -- to send your son to your in-laws' home. Perhaps it will also help you to know that it won't hurt him psychologically to have different rules at different homes -- as long as he's safe and loved.
But this goes beyond babysitting -- it's potentially a marital minefield, too. I strongly suggest letting your hubby deal with his parents on the childcare issues. It will be much easier for him to deal with his parents on this one.