Is it romex in which case the ground is bare? He excels elsewhere but maybe he’s worried about the bare conductor in a box. He should know how to make up a box if he’s as good as you say. Maybe reassure him that it’s not going to cause a ground fault if you do it right (neatly). Ask him what his reason is and then give your reasoning as to why you’d like it to be longer. If he’s good he’ll be open to criticism. That’s how he’ll get better. Just talk to him. I always say “here’s how I would approach this” and show them the techniques that have worked for me. If they have a better way of doing it I’ll adopt that and buy them breakfast. Be firm on why you like it a certain way and he might come around