I'm sorry because I know this is not what you want to hear but I honestly do not think you can (or should) insist on what gifts someone does or doesn't buy your kids. Especially loving grandparents (I'm giving them the benefit of the doubt that they are your normal standard loving grandys).
Two main things stand out in your posts. Firstly I truly do not believe that the multiple gifts that grandparents give will cause your kids to be spoilt, materialistic or ungrateful. It simply gives you many opportunities to teach your son how to graciously accept a gift. Start young in insisting he says thankyou, opens the card first and calls or sends a note to say thanks for special big gifts. You are currently being ungrateful for the gifts- yes I understand you feel that you have a good reason not to be grateful, but perhaps consider how that will look to your son as he grows up.
Also you say that they do this for all the grand kids? Well seriously..... Do you really expect them to have a big family christmas celebration and to have all the cousins open 5 big presents each and then for them to give your son a tshirt?!?! Your son will defiantly notice this as he gets older and that will very likely cause negative behaviors and emotions. More so then the gift giving IMO.
I totally get that parents and in-laws can be very annoying and overbearing when you are learning to be a new parent but I think in this case you should take a step back and look at the bigger picture. I agree that having a relationship that completely revolves around your son receiving gifts is not great so it's up to you to help foster a loving, intimate, fun and playful relationship that your son will naturally value over the stuff that they buy. Pick your battles I say. For example I think I would rather my mum give my son a small gift whenever she sees him rather then give him heaps of junk food to spoil him. That's just me.
Good luck. X