Get out the PTS and give him a simulated check ride. A few slips, that he catches himself and corrects, okay. The tolerances aren't as tight as your CPL or IR, so don't expect that. Remember,you don't have to do every maneuver and can combine tasks.
The PPL is just a license to learn, independently. Is he safe? Is he still willing to learn? Rank 'em 1)attitude, 2) knowledge, and 3) ability. Somehow I've always found these endorsements easier to give than when the teenager wants to get a driving license.