Lloyd,
DO NOT set him up with the little "Youth" version of the Mossberg 500, the one with two stocks and an 18" barrel.
I have one here, and can report that I believe the gun was designed by activist PETA members, in hopes of discouraging youth from pursuing hunting.
The smaller of the two stocks is a torture device, that fits no-one. The 18" barrel lets a whole bunch of "Real Loud" out the end at the shot, and if shooting near dawn or dust, about a foot of flame.
I have one, that I put the longer of the two stocks on, and found a used 26" non vent rib barrel for, from a much older 500 20 gauge. The lug needs to be relocated, and when that has been done, I will have an actual "Youth" Mossberg that will be a far better first time gun.
CB posted a fine gun, but, I believe the Remington model 17 to be a superior choice to a model 12, in any kid catagory one might think of-size, weight, cost, anything.
Should I bring my model 17 and tell him you had one just like it, and that it would have been dandy for him, but, HIS DAD SOLD IT?

Not even I am that big a dink.

Good luck, in any case.

Best,
Ted