At 12” LOP, you have your choice of any youth gun, so pick the one that fits and balances. Wouldn’t hurt to look for one that you could buy a full length stock for, as he grows.
@Nick87 is spot on with shell distribution. No matter how many it’ll hold, he can only shoot as many as you give him.
On a side note, I started my boy with a .410 with a red dot for turkey. The .410 is great for that, but the red dot is unnecessary. When he moved to a 20, we patterned it with the bead. Let him do the whole thing and come to his own conclusion of how things worked with aim and point of impact. It worked out well.
Now he shoots an older cz 720 (with a dad modded stock) for everything. Averages 18 in his trap league, wing shoots well, and has a couple turkeys “old school” aiming with the bead.
Long story short, pick something you like and think will work and let him practice with it. Lots of fun.