They are very good bikes, but not really the best for a learner (assuming your son is?) to be honest. At most, you're looking at a rebuild every 8000 miles (and that is absolute tops), although being a two stroke that will only cost a couple hundred quid. You also need to warm them up every single time before you start riding them, use proper (AKA expensive) two stroke racing oil for them as well, and they properly suck petrol down. They're also pretty uncomfy (coming from a pretty flexible and fit 21 year old), and do not work in the slightest with either pillions or luggage. If you are looking at one, odds are it will have been owned by a bunch of kids that won't have a clue how to look after it, so don't buy unless there's evidence of a very recent engine rebuild.
However, get a decent one, look after it, and you'll have a great handling bike, that will be pretty damn quick when it's derestricted. I'd still say though that two strokes, particularly highly strung ones like the RS series.
To be entirely honest though (assuming your son is 16, hence why he wants a 50), unless you wait until you're 17 and get a 125 (which there are a great deal of about in various shapes and sizes) it'd probably be better off getting him a cheap scooter to nip around on anyway, or possibly some kind of 50cc dirt crosser type thing - I can't think of any 50cc sports bike types, although there are a few 125s if he's only a little bit away from turning 17 and can wait?
