Today it was warm enough that I needed to pull out the shorts for Ds (almost 22 months old). I had bought a bunch of shorts through the spring, in sizes 18-24 mo. and a couple of 24 mo. sizes in the brands that typically run a bit smaller (like Jumping Beans). He's been wearing 12-18 mo. size jeans from Old Navy, but they were getting a bit short (just barely), so I got him the same style, but 18-24 mo. They wouldn't stay on, but then I noticed that they were adjustable, so they will work.
Back to the shorts...Ds has been in 12-18 mo. size clothes from Gymbo since this winter. So I put a pair of 18-24 mo. size shorts on him and they literally fell off. He is so skinny and tiny. Since I didn't have anything else smaller for this summer and since it was 85 degrees, I tried a pair of 12 mo. shorts from last summer on him and they fit just fine, both in length and around his waist.
The kid eats a wide variety of stuff and is pretty active....would you be concerned that he hasn't gained enough weight to since last summer to outgrow his clothes?