I have a 14U switch hitter. Natural RH. Throws RH. I must admit I'm a little confused by the original premise. Am I reading it wrong or are you suggesting he goes full time LH hitter & not switch? Why would this make sense at this point? Seems like he is managing well RH & may have an advantage (hockey) to looking into hitting both ways. Why would you eliminate his natural side?
If you try it, here would be my suggestions having worked through it starting at 9YO.
#1: Just go for it. Everyone is going to be against it with a few exceptions. Get the coaches buy in & forget the rest of the naysayers.
#2: Start his LH at bats in low pressure situations. Pool play games that are a blow out. Rec Ball where nobody is REALLY concerned with winning.
#3: He needs to play REC ball & go full time switch just for the low pressure AB's.
#4: Starting out, ANY contact LH is a victory. Be patient, he will struggle.
#5: Tons of live BP both ways. ANY hard hit ball LH is cause for celebration.