RE: Best Board for Young Beginner
This is just a quick answer, as I'm "in between trips" at the moment.
I think I would suggest either the Rio S or the GO 139 with a set of the
12 year olds that weigh 45 Kg. (99.2 lbs.) tend to grow quickly.
The Rio S is 150 liters, and has a centerboard so he should be right at home on this board and progressing quickly.
He will probably be planing and in the straps in a few sessions.
The Rio S is pretty good a an early planing short board, so he has that to look forward to, Just retract the centerboard as soon as he no longer needs it in planing conditions.
If you prefer, the GO 139 can be used with the side fins until he'a planing, then take them off and he will have a great little shortboard that should last him until he grows to 65-68 Kg. when he will need something larger for lite winds.
Hope this helps,