Crimson, I am new to your situation, and I am sorry for the emotions your son is going through. I don't know what the custody arrangements are, but it sounds like you are both part of his life.
Does he look like he is having fun there? She may have sent the pics to let you see him having a good time.
We aren't physically separated but we don't go anywhere as a family. I don't pursue, and the few times I do, it is shot down. So either the kids go out with W, or with me. And D5 is picking up on that. When I take them somewhere, she says, "Mommy isn't coming AGAIN? That Mommy."
You brought it up once, maybe if you bring it up again by a slightly different route and then drop it unless/until you see this behavior again from S? Maybe he was just having a bad day.
You sound like a very good Dad. Let that encourage you.
_________________________ Me: 37 W: 37 M: 11 D:5 S:2 IDLYA, W removed rings, BD 07/13 EA/Fantasy (PA?) confirmed 12/13 W moved out 05/14