Hi, I just noticed this post, I hope that I'm not too late to add my 2 cents worth. My daughter is 4 about to be 5, and wears 6 pants, and some 7 in dresses, pjs etc. She is very thin as well, and I have a hard time finding things small enough in the waist that are long enough. I have found some things that work for us - hopefully they will work for you too.
1st) someone else suggested my first thought too - overalls. You can buy them extra long, shorten the straps to the shortest setting, roll up a cuff, and when he grows and the cuff comes down, then start letting the straps down. They don't have to fit around the waist so much, just put a longish shirt in there and they are plenty cute! Oshkosh always has specials, and online coupons that you can print, there are outlets all over the place.
2nd) sweat pants, I've had good luck with those too, having small waists, because of the elastic in the hem too - you can buy them a little extra long and they can just be full at the bottom, he won't trip because the elastic will hold them up.
3rd)I NEVER put anything in the dryer where length is a problem - because even on the coolest setting I think cotton things continue to get shorter (I'm very tall, and I know that I'm not growing and my jeans get too short if I dry them) I just put them on hangers, and hang them on shower curtain poles, or the back of closet doors, etc. Pjs - almost always last about a year for us, because I buy the cotton jersey t-shirt material legging and shirt style, and because they are close fitting, I can get them extra long too, and NEVER put them in the dryer.
4th) consignment stores - usually a selection of inexpensive pants - I found a Ralph Lauren pair, paid only $7 for them, they didn't have an adjustable waist, but it was tiny, and they were super long, I had them cuffed at the botton 2+ times, she fit them forever! When they finally started getting snug around the waist, she lost the diaper/pull up and gained that room back!
5th)in Orlando area - there is a store "Steve & Barry's", if you are in the area, seek it out, if you're not in the Orlando area, check online and see if there is one near you. EVERYTHING (adults & kids, shoes & clothes) is $20 AND LESS! The kids pants have adjustable waist bands, and are only $10, they fit my daughter like a dream and aren't as expensive as GAP, Gymboree etc....
Good luck, let us know how you make out!