My DD1 was similar at that age - around the 10th for weight, 50th for height and 90th for head!! Just like her father - tall, skinny with a big head Anyway at 6yo she is still tall on 75th now, still has a big head 90th and still skinny for her height but is now at the 40th.

If your DS is happy and healthy and you have a certain build that runs in your family then I don't think there is anything to worry about.