Is there someone he wants to do the MX with so you can share a trailer with?
Most van hire places you would need to hire for the weekend and you're probably looking about £100+ for a small van.
We bought my old Jeep for the quads, little ones fit inside then the trailer for one of the adults ones. To be honest a lot used just a mondeo or astra or something with a trailer on so couldn't see the Lexarse being an issue. Most tracks have a service road in/out for cars/vans/trailers.