At 12, your son is not ready to change arm slots. He is not even throwing from 60 feet. A lot of guys who do not have great veloicty drop down to steal a strike out or two, but in order to do that you have to throw at least a fastball and breaking pitch from that arm slot.
The are a number of big league pitchers who you will see do this, Bronson Arroyo of the Reds is a good example. He will throw one curve from over the top, then a one from sidearm, but he has great control. A sidearm curve left too high is a homerun pitch.
The most difficult pitch to hit is the down breaking curve. At 12 he is not ready for that either because when you lead with the heel (bottom) of your hand, you stress the outside tendon of the elbow.
Work on his fastball 2 and 4 seam varieties, then a change up. When he can locate those with command, and he is older than 14 1/2, then look at a breaking ball.
Changing arm slots is at best a late high school undertaking. Also, I think you have to have a natural knack for doing this, and be willing and able to practice it A LOT. There are guys who have been successful with it, but they are the exception to the rule.
Warren Spahn never believed in changing arm slot at all.
Good Luck to your son,