From your post, it seems like you may be searching for a size too small? My son was very similar to your son's size, and has always been 2-3 sizes up from his "age group" he was wearing all 2T clothes at 12 months. Now, at 18 months he is moving towards a size 3T.
I know the adjustable waists (with the buttons) don't help in our situation - he just is always at the last button! I was kidding with my husband that they need to come out iwth some "husky" sizes for the toddlers!
I have had the best luck with Baby Gap, Gymboree and Crazy 8 for pants. These stores I have been able to find 2T with at least a half or full elastic waist - which is what I always try to find. We have a lot of overalls too- those you can buy big and roll up.
Good luck! I hope for both you & I that ours sons hit the "slimming down" stage soon! :)