If that XT isn't beat to shit and the price is reasonable, snatch it up. Terrific little bikes. Great on the street, and if your son wants to get into serious dual-sport riding, a few suspension upgrades should make it really good in the dirt too (or if your son doesn't weigh much, it could even be fine as is).
Here's my old XT, although by this point I'd sold it to my best friend:
--mark