Hi R.,
Your parents-in-law aren't aware of the problem, and no amount of subtle remarks or motions will probably change that. You and your husband have the responsibility for setting firm and reasonable personal boundaries or accept the situation as it is. A boundary means, in this case, some kind of limit to the length of time they can play, cuddle, hand around your son. On the other hand, you don't want your son handed around to everyone...he'll grow up to hate family gatherings. Remember Aunt Marta with the red lipstick. Yuck.
Setting reasonable boundaries and limits are good things and that people have different ideas about boundaries. Boundaries are how we protect ourselves from another person coming too close. It's important for your son to learn boundaries and that you'll protect him from hurtful situations. Some families have no boundaries or limits; some have too strict limits, and some are OK. It takes time and practice to believe that you deserve boundaries. It's important to set reasonable boundaries if you're not used to having them, especially when people try to breakdown or step around or over your limits (which they may be unaware of because of their personal history). Some people set boundaries that are forms of denial -- they won't hear anything that would conflict with what they want to hear.
Brainstorm with your husband about what you really want -- how long and when his parents could play with your son, when it's OK for your father-in-law to tag along. Write it down and keep it handy. It's best to act consistently.
I believe in telling people that you're going to set limits, but in this case it depends how self-aware your parents-in-law are.
You could try this: When you're alone with them, tell them how much you appreciate their attention to your son and how much he loves them. Then you could say, I don't think that you've noticed, but members of my family would like to see him too, that you'd like your son to be comfortable with other people.
They may react in any number of different ways. You can practice the broken record - I understand that but it's important to me... over and over.
Now's the really hard part. When your parents-in-law come in and try to take the baby from Aunt Mary, say something like, "Oh, sorry, Aunt Mary has been sitting patiently to hold him." Keep to your set limit, be flexible, but you must be as consistent as possible. When they have him over your limit, collect him and say, Aunt Rose wants to see how big he's grown. Be careful of your son's boundaries...
Is your name Rosalind? That was my mother's name. I really like it because it sounds romantic - like Rosalind in one of Shakespeare's plays.