Why does it bother you that he stays in his room? Is it because you care about him or you just feel superior to him because you and your daughter are more "independent"?
Because if you cared about him (and I get it, my 11 year old isn't adventurous at all and I never stop finding ways to get him out of his space), you, or more exactly, his father, would find ways to figure out why he doesn't have motivation. For example, I'm now bringing my son to a psychologist and we discovered that by not going things, he's protecting himself from failure, etc.
The reason you're getting hard time here is the tone of your first post. And I'm seeing that it's a bigger issue than just the vacation, but people don't know it.
Last summer, I was suffocating with SD who was constantly telling me what to do with my baby as if she were his mother and I was some babysitter. Not to mention that she would do things, like coming to my DP asking to dance with him in a middle of our dance, after several dances with him and after telling me few minutes before that she was tired. I wanted to be nice with her, so I didn't correct her and as a result, I just couldn't stand her being in the same space as her at the end.
But in your case, it seems that the boy just doesn't have motivation to do things and it's annoying, but calling him a "limpet" who "threatens" to come with you just doesn't sound nice.
Well, he will go to the Uni in Sept, so you'll have plenty of possibilities to have drinks with his father.