Some of the boys' tshirts come up pretty large. My 5yo is gigantic (128cm) but skinny and I have him in Boden 7-8 tshirts - the short sleeved tshirts are very slightly on the large side for him, but OK. The Boden long sleeved tshirts in 7-8 are just wearable but they are pretty long. So prob go for 7-8 as 5-6 will be too small. There is no 6-7 in tops, but there are more sizes in trousers.
For shorts, you could probably get an age 6 for a tall skinny 6yo. However, watch the reviews on the waists as if he is skinny, some stuff will fall down!