Maybe that manager is jealous... He should realise that anybody would take a f/t job over a p/t one. How is your son ever going to get a mortgage, or even pay rent, if he keeps working p/t?
What's the turnover in that company? As you said, they wouldn't hesitate to let go of him if they had money problems, so there isn't any reason for him to feel guilty, least of all over the training he received. I've asked employers to give me training over and over again, and even if I stayed there several years, I never received any. Basic in-house training, yes, but anything like IT courses, accountancy courses, professional qualifications, I've had to pay myself, while colleagues who were less the managers' friends were sent on courses to learn how to create a text box or to put their out of the office on!
Good luck to your son in his new job!