Our TT sleeps 10. That's why I bought it. DD always went with us, and then filled the TT up with grandbabies. DS couldn't go, because he said there wasn't room for him and his family. I fixed it. Now there is.
Thing is. With that many people in a TT, You got to have some ground rules. And you got to tell everyone about them BEFORE you leave on the trip. We do, and every one is to be sure their kids go by the rules. Simple things like, No running in and out, staying out as much as possible, and using the campground bathroom as much as possible.
It seems to work well.
So setup some rules. Tell him they are not optional, and IF he wants to stay in your TT, He, and his family must camp by the camper rules.. I would also tell him, that IF there is an extra charge for more people he will pay that, and in truth. He should pay at least 1/2 of the campsite fee, and grocery bill.
ONE of our rules is. There is only 2 reasons to go into the camper. You go in to eat, or got to bed. No other reason to be in there.