Unfortunately, I think the onus is on you to provide food your child can eat. I have friend and family who are GF and they all bring their own things. As someone who tries to be an exceptionally good hostess I try to also prepare things that are GF. For example, when my kids' friend who is GF comes over for one of their summer parties, I make sure I make him some roasted potatoes, serve regular potato chips, have plain grilled chicken and salad for him, watermelon, and GF ice cream. Of course I make these for other guests so he's not singled out, but I make sure that he knows what is GF. That said, he and his mom don't expect this and before they really trusted that I "got it" about being GF, he would usually eat before coming over and only munch on items that he brought. We had several GF family members at Thanksgiving last week and the mom in that family prepared GF versions of several side dishes and desserts. My GF aunt does the same - she brings something of substance that she knows she can eat as an entree and share with others (like chicken kabobs) and then rounds out with any foods that are obviously GF, like plain potato chips, salad without dressing or croutons, etc.
If I were you, I would do as you are planning and would bring GF versions of his favorite sides and a dessert that he can eat, but bring some to share if possible. The more others eat GF foods, the more they might be willing to make some when they host. For example, I made my GF aunt a birthday cake last week and made part of the cake from a GF cake mix. It was delicious and I would readily use that mix again (King Arthur GF Yellow Cake Mix, by the way).
Given the complexity of your son's dietary needs, it's a bit unreasonable to expect others to be able to plan a holiday meal and accommodate those needs in addition to doing everything else. With something as important as the food your child can eat, I wouldn't trust other people to remember everything or check every ingredient either, so I would feel more comfortable knowing that I brought items my child could eat without worry.