It totally depends on what he wants to do. Our son, Peter, plays keyboards and anything else he can lay his hands on. He is into the sound something makes and at the moment that is electric. He can play (untaught) my acoustic guitar (or mandolin) but he wants a particular sound. In his case we found (at half price in a sale) a twin necked 6/12 string Tanglewood electric (just under £200) to which he's added a Line6 pod. It makes wonderful music (and noise) and is in tune with what he wants to do. It helps that as an ex-chorister he has the basic musical knowledge so he doesn't need formal lessons but then I never had music lessons either and play too many instruments as well.

To me the important part is getting that spark that so they want to learn, play, improve. If they don't want to play an acoustic guitar then no amount of cajoling about how it will helpp them later wil work - unless you bribe them "get to this standard and we'll buy you a Gibson/Ibanez/Fender" (delete as applicable).

All the best

oggie