What I did for my son was get him a small women's board at that size. 139 If I remember right. They have the ergonomics and build for his size. I just found one with ambiguous colors/design. I think it was a Rome detail. Shortly after we switched to all smokin snowboards, all the time. Their hooligan was a great all around board for him, stiff enough for big mtn and big jumps and flexy enough for hitting some urban stuff. Make sure you get something with as much edge control or more, nothing full rockered unless that's what he rides now. The kid won't know why but he will not like the wash out effect of the rocker and will get away from that board.
Anyhow. He needs a real snowboard and not a "kids board".