That is small for a Northern, but many things could be responsible. Are you sure that it's a pure Northern? Many, if not most Northerns in the US are no longer pure-blooded Northerns. Assuming that it is pure, they will grow much slower if they are fed less during the first year. Even so you would expect most to be 19-20 inches min by 1 1/2 years. As long as he is healthy and not losing weight, you really don't need to worry about him. I have not heard of a skink growing much after 1 1/2, so this is probably as big as he's going to get.