The Mizuno Prospect Series (GXC112 Youth Catcher's Mitt) will have a bit of a smaller pocket than both the Shoeless Joe Professional Series (3200CM Catcher's Mitt) and the Shoeless Joe Joe Junior Series (3000JR Youth Catcher's Mitt) even despite the smaller circumference size in the Joe Junior Model. That being said, all 3 mitts will similar in the softness of the leather and require little-to-no break in time. All 3 mitts you are looking at are excellent for game-ready catching options.
The Shoeless Joe Joe Junior Series (3000JR Youth Catcher's Mitt) is a youth model at 30". It would be a great size for a 6-8 year old player.
At 30.00", the Shoeless Joe Joe Junior Series: 3000JR Youth Catcher's Mitt may be a little too small for your son. I would look at a mitt closer to 31.50" in size.