Both boards would work the same for your son. The average 12 year old would not feel any difference between 151 or 171 liters. The only one who would feel the difference is you. The 151 would be less stable then the 171 at your weight in light winds but nothing you couldn’t handle. In higher wind planing conditions the 151 would be less bouncy so it would feel more stable. So the closer to 20 knts you sail the more you would prefer the 151 but the closer to 10 knts you sail you would prefer the 171.
At my school we use Starts, Rios & Hifly Primos which are 70 cm wide longboards. For students first time on a board we use the starts. Learning to uphaul on a start is so much easier then a longboard. You can also learn on a long board but the Start is easier for everyone. Not some people, but everyone. Learning to do your first turn on a board is also much easier on a Start then a longboard. Again, some people take to the sport more naturally then others but even those students learn to turn much easier on a Start then a longboard. So our procedure is to teach basic uphauling, turning & their first sail on the Start then transfer them to a longboard within their first hour of sailing so they get better glide & upwind performance. When it comes time to teaching them their first tack & jibe we go back to the Starts for the basics & first attempts then transition them to the longboards.
On the longboard shortboard thing. That is a real tough call especially because many people are not honest about the wind they sail or plan to sail in. The Go is meant for light wind planing not light wind schlogging. If most of your sailing is going to be off the plane-schlogging -then the Go should not be your board. Get a longboard. If most of your sailing is going to be in light wind planing from 15-20 knts then the Go would be a great board. The 171 for closer to 15 knts & the 151 for closer to 20 knts.
There are pros & cons to every board and all of us have opinions on which is best. 1-21 knts is a big range & I would find it hard to believe the wind would be equally distributed in that range. Once you truly know what wind speeds most of your sailing will be done then the choice of board will be easier.