How can I un-invite my sons girlfriend from a family holiday?

[Bumbelbee]

They are both 17 and have been 'dating' for a year. They live quite a journey away and see each other for a long weekend 1-2 times a month. He asked if she could come with us on holiday to France so we said OK. Now he's decided perhaps it's a bit too much as she isn't very active, seems to prefer lazing around to anything else, playing internet games. He wants to do things like swimming while we are away and realises he'll probably spend most of the holiday in their room if she comes with us, and there's not much point in being away. He has a serious medical condition and know he needs to exercise regularly as well, swimming being particularly good for his health. He has asked me how to tell her she can't come. I have no idea, I think she'll be upset. I can take them away in the UK later in the Summer so she would get a break but it's not the same. I feel it has to come from him not us really, but is this reasonable? Any advice?

[Whathaveilost]

He is two years older than DD who obviously wouldn't be allowed bring a boyfriend away with us, but, if she created a situation where she decided to let someone down, she would be handling it herself.

Why obviously?
My DS who is now 16 went away with his girlfriend's family when they were both 15. What exactly are you saying here?

[Whathaveilost]

My bold didn't work. My response was to MaisieDotes

[P1nkP0ppy]

If this is potentially the end of the relationship and she does go, you will need to be prepared for the problems if the breakup happens while you're away.....