Well I'm basing my recommendation off the assumption that he will be doing some sort of freestyle. The Proto CT is a true twin after all.
If you're looking for big mountain carving goodness, there are better options for you in the NS line.
If you are heavily park focused, there is the Evo.
The Proto is going to be a great choice for all-mountain freestyle. The true twin shape gives it more bias towards the freestye aspect than the setback SL. But the flex of the Proto lends more to the all-mountain side than the Evo.
I haven't been on the SL, but all the specs point to that being the better all-mountain board.