Yes, I see. I thought that it might be a site licence problem.
Your wriggle round solution is clearly your son's purchase of the caravan. Make sure that you have a private written agreement with him as to the actual position. I have advised you of the possible problem which, I would hope, would not be a problem at all. I'm an accountant so naturally a belt and braces man and view everything with deep suspicion!
I think this is the best, XXXXX XXXXX most economical solution.