I never paid too much attention to weight for the young kiddos. As long as they are eating (and a good variety, not just one or two foods) then I didn't worry. My kids were always on the heavy end (not fat, just SOLID). When my daughter was 1 yr old, she weighed more than a 24 month old at our church. But, that child was feather light! Seriously--I went to pick her up one day and almost tossed her, because I expected her to weigh much more than she did. Her dad picked up my daughter and was shocked the other way--because my daughter wasn't "fat" and so he didn't expect the heaviness (plus he was accustomed to his daughter, who was SO light).
I would definitely talk to your doctor if you are concerned, but if he eats normally, then he probably is just fine. As far as clothing issues, most kids' clothing comes with the elastic hidden in the waistbands where you can adjust it with buttons. Look for that when you shop for his clothes. My son turned 13 this summer, and finally had to start buying "men's" pants. (After his latest growth spurt, however, he is back in his 14's and 16's, lol). No more hidden elastic in the waist!! Yes, some pants go all the way thru size 14/16 with those hidden elastic bands.... so you should be fine if you just look for it. It helps a lot!