I don't think, unless you have no fence or physical barrier between your two front driveways, that he is allowed to access his house from your drive. It may be worth approaching him to ask (politely and calmly) if the caravan is a permanent addition, or temporarily being kept there (pending a holiday, or something). if it's to be permanently stored there, you will have to ask him not to come onto your driveway to access his home and let him know that this is unacceptable. Keep a note of what you say, and the date and time you say it. If that doesn't work, then you'll have to let him know in writing that you are not prepared for him to do this (and keep a copy). if he still does it, then contact his letting agent or landlord.