Your son would probably want to get something between 135cm and 155cm. I don't know much about youth skis but he is right in between the kids park skis and the ones that most older people use. One ski that come to mind is the Tom Wallsich shorty which has a reputation as a good ski.
It might be a bit short but it does also come down to preference -- and this is an amazing price for a 2017 price. Armada and Line are probably the two big brands I would recommend but I'm sure there are plenty of people who know more about skis than me.