I rarely recommend cartridge razors, but...

I would say mach 3 is the best option when we are talking about cartridge razors.
I also would say the big differences between traditional and modern wet shaving is in the brush and cream/soap and the canned goo.
P.s i think you did the best suggesting a cartridge razor.As someone stated before,wet shaving is not for eveyone.
 
First shave of my 17 yo son (when he was 15) was with a DE, Cella soap and Vie Long horse brush. But I've turned him to a disposable (Gillette Guard razor) to make his life easier. When you're 15 your time is too precious to slow down... If in the future he will switch back to a DE, it will be him decision. Now he's using the Guard once a week with Proraso White and a synth brush.
 
Sure you did. That was very sensitive to pick up on his development issues and guiding his mother in that direction I think was darn good advice.
 
I think you did the right thing.

It took me ages to master DE shaving, only after studying YouTube videos, did it click for me.

From memory, the moto of surgeons is do no harm. The gentleman concerned has options, he can always move on from cartridge shaving.

All the best,

Chris