The question of why you are transferring comes up in the business world all the time.
The prospective employer may say:
"Why are you seeking to change jobs?"
Response 1: "My boss is a knucklehead and the management at that company are all cheap skates" (Exaggerated for effect).
Framing things negatively will give the impression that you are a problem employee and you will thus very likely not get an offer from them.
Response 2:
"I really like that company and the opportunity that they gave me, but I just don't feel challenged enough at this time and therefore I am seeking an opportunity for professional growth like you are offering."
I advocate truthfulness but my advice is to frame potential loaded questions in the positive and that will hopefully prevent any negative red flags from getting raised.
You can practice these skills with your son as well. Pretend you are the coach and ask him all the potential questions or concerns a coach might have and practice how to frame the responses in the best possible light.