Hi L.,
I just wanted to respond to you to let you know you are most definitely not alone! I know how hard it is to hear some of those things. I can tell you this, I am still in the process of divorcing my children's father and I am their primary, however, they do the same things your son is doing. I can't talk about birthdays or plan parties or anything without hearing about Nana, Grampa, Daddy and so on. It can be soooo frustrating and there are moments that I just want to say "I don't care!" but I think that would just make it harder for them (mainly my daughter, she's 4, my son is almost 3 but isn't as obsessed with his other family-perhaps because he doesn't get to spend as much time with them, i don't know.) I've gotten into the habit of trying to at least sound truly interested in what they are trying to tell me about the other family and continuing my conversation without letting the focus move entirely to the other family. I figure if I don't show my frustration and just let most of it roll right off me then maybe they will grow up feeling lucky that their mom was so cool about their situation. It does get hard when it feels like they don't appreciate the things you get them or the money spent, but luckily I have a good support network around me to remind me that the quality of time we spend is going to be so important in the end. I'm sure as my children get older I'll continue to experience some of the things you are describing. I don't think any bit of divorced parenting is easy that's for sure!
We all grow up remembering a few random cool toys we had, but more importantly we hold on to the way people care and how they make us feel and ultimately I think we recognize the time and effort put into helping us grow...so as hard as it is to hear about "the others" (as we've termed them in my family, lol), it's definitely good that he feels comfortable mentioning them to you.
I know for me, I'll never know what their little minds are being filled with and I just hope for the best. And when they slip up and say something hurtful I let them know. Something I've recognized from my children occasionally is what seems like an issue of loyalty. (I'm not sure if you've seen it that way from your son?) I can tell in the way they act when they come back from spending time with the others and I usually give them some space to adjust and then explain to them (at times when they are hurtful and need to be reminded) that they can love everybody and that they are lucky to have so many people that love them.
I know we are experiencing things from different angles, but it's funny how you get the same thing on both sides isn't it? He may even act differently when he goes back with his dad after having had time to hang out with mom. Try not to stress too much. Just keep being the best mom you can be and he'll recognize the effort when it truly matters!