Trust The Krink to be 'helpful' !

As Charlie said , it depends what sort of music your grandson wants to play ! For Bluegrass , a five string , preferably with Resonator, for Song accompaniment , an Open Back is not so loud , and for Irish Tunes , a four string .
My personal belief is that , for a learner , a GOOD used is a better buy than the same priced new , as long as the neck is straight !