I'd guess that A LOT depends on his age, size and skill level. By the time you bought a used trx 90, cr85 engine and all that other stuff, plus rebuilt it all(like motor, bushings, bearings etc..) you prob wouldn't be to far off from say a DRR or some other purpose built quad, which would prob have a higher resale value when the time comes for him to step up again. It would prob take some time to find good deals on that stuff to lower the cost some, plus IDK how your fab and motor skills are(not being rude, just that IDK).
I've been thinking abt getting my little girl a quad too for some dune trips. The only thing that seems like it would get around decent at LS would prob be a lt80 with some mods of course. She's abt to turn 6 btw and already has a crf50, which she is learning pretty quickly on. Not to hijack your thread, but does anyone know how well these get around in the dunes? I just don't see a 4 poke that's 90cc having any power to climb or do much of anything, plus the suspension looks chitty on all these machines IMO, but I could be wrong. Just not looking for another money pit that she'll(or any kid) will prob outgrow fairly quickly.