I can see a preliminary session with a dull razor, just to learn the different grips, angle, and pressure, but one session only, then immediately on to a proper shave ready razor. A dull razor can quickly develop and encourage bad habits.
You need styptic on hand, and he needs to understand that it will burn but will stop minor bleeding quickly. Yeah, he is probably going to cut himself a few times. Summer is a good time to do this. Send him to school with shaving cuts and someone will gasp in horror and call child protective services on you for allowing your son anywhere near a deadly sharp object like a straight razor. OTOH, a good shaving cut or two from a straight razor will set all the girls' hearts a-flutter and make all the boys envious.
I can only imagine what would be said if he shaved with a straight after gym class lol.