C.C.
I go by the child's measurements, and the manufacturer's size charts. They do tend to vary quite a bit. I've never gone wrong using the child's largest measurement. For instance, one of my daughters has a large "girth" - the measurement that goes between the legs and over the shoulder, measuring the torso height, basically. So I size based upon the girth measurement and not her waist, hips or bust, which are small for her height. As kids grow, their measurements change so fast, so just take out your tape measure, measure his height, chest, waist, hips and girth, and then compare those to the manufacturer's size chart. He may wear a 5 in one manufacturer and a 4T in another.