S.S.
Hi M., I was really interested in reading these responses because I too have a tall/slim son. My poor 6-year-old; his pants either tend to be high-waters or they slide down his (nonexistent) hips/butt!
A friend recommended Walmart to me; they have Wrangler Slims and Faded Glory (both with an adjustable inside button), and they're not as expensive as some.