Erm, definitely worth it. It's not all about power, and if he's on a light bike he'll be nice and fresh when it comes to picking your bike up
I think most of the manufacturers do some sort of learner legal trailie or at least something that looks like one. I'd just look for the cheapest thing you can find which has the right size wheels for off-road tyres.
That said, there was a lad turned up to the second day of the Cambrian in August on a 125cc Yamaha with road wheels and tyres (XT125X? WR125SM?). He made it round at least one lap, which was more than his dad did on a BMW G450X
Don't know who you'll find to hire a bike to someone without a full licence. If you can't find one to buy it might be best to go with one of the training schools out there - I think some of them are able to run courses entirely on private land, which makes the licence thing less important.