Hi edlo84 - as a DIL myself with difficult inlaws I hope I can be objective. But you seem like you really are taking a hands off approach. And I totally understand how that would sting that they celebrated with her parents.
Could it be that your approach is too hands off? I was thinking that if I was in your shoes I would have said to son, "well your b-day is coming up, I'd like to take you out for dinner, what would be a good day for you?" That's a no pressure approach in my opinion. My inlaws use this approach, which I appreciate. This is one of the few times we don't feel guilted by them. My inlaws are covert-aggressive and my MIL never puts on direct pressure. MIL is NEVER direct, that's not her style at all, she always uses guilt and "oh feel sorry for me and please give me some attention" approach.
The only thing is when we do go hubby and I both feel uncomfortable because MIL tries to pay for everyone's meal but that makes FIL mad. I guess he doesn't like that they pay for my meal as well and he makes comments like, "momma are your gonna pay for everyone's meal?" And I think to myself well the last time we took everyone out for mother's day, FIL tagged along and WE paid for everyone's meal including HIS. But that's just another one of their covert-aggressive techniques. They resent having me tag along for hubby's b-day celebration.
I wonder if your son and DIL wonder why you didn't mention anything about his b-day. I assume you're not having any trouble with your new DIL. If that's the case I just don't think they would mind if you took a little bit more of an aggressive approach. Just my thoughts. Good Luck.

One more thought about this, is now whenever my hubby and I meet them for his b-day lunch, which we always feel obligated to do, we arrive about 20 minutes before and pay for both our own buffets lunches so this won't give FIL an opportunity to try to embarrass me. And no mother's day lunches or anything like that because he just has to ruin it everytime. Sometimes the good thing is that if we don't go to the restaurant that he wants then he usually just doesn't show up.