Do you have a good enough relationship with your parents that you could ask them what they would do if you put up a Christmas tree in your own home? That might be a good way to phrase it; it's your home, not theirs. You could make clear that you are not asking their permission. You're an adult now and you know what they think of Christmas. You don't agree with them. What you want to know is where they would feel obligated to draw the line. Would they take steps to have you DFd? Would they shun you? You'd like to know where you and your family (including their grandson) would stand with them.
If you don't feel comfortable even talking about it with your parents about this, then you'll have to base your decision on your best guess as to what they would do and decide whether it's worth the risk of getting hit with the worst case scenario. I personally don't think I would do anything behind their back. That doesn't mean you have to invite them over to see the tree and exchange gifts, but neither would I want to be sneaking around as if I were doing something wrong.
Even if your parents agree to look the other way, you would still be taking the risk that your son would tell JW kids about celebrating Christmas. Their parents might then turn you in to the elders. Only you can decide if the risk is worth it.
Regarding Santa, one option is to tell your son that Santa is a fun pretend person like Barney the dinosaur but that some kids think he's real so please don't spoil things for them by telling them he's just pretend. They'll find out soon enough. We took this approach with our kids (we didn't have the JW family issues to worry about). Our kids enjoyed Christmas just fine and we never had to tell them that we had lied to them about Santa.