I would like to think that an 18yo would be mature enough to consider the needs of others as well as his own. Maybe it's time to have a chat about the bedroom arrangements and see what he suggests. I've had friends in similar situations ie having to share a room with a sibling and needing quality time with the GF, and they have taken turns camping out in the lounge for the night.

I also know of several friends that moved out of home due to not being allowed to have the GF/BF stay in their room so I think he should also acknowledge you allowing him to especially with young siblings.

The caravan option is a good idea, though they can cost a bomb to rent a can be comparable to a unit. MIL did that for BIL for a while and still carries on about how it cost thousands.