My son is a string bean too (he was still wearing sz 4 shorts at 7 years old).
I found plenty of adjustable waist stuff at thrift stores and consignment sales. I started bringing a current pair of pants that fit along with me so I could measure the waist and length. (made such a big difference!!).
For brands, Children's Place and Old Navy were usually OK. For Old Navy, it wasn't good when he was younger, but once he got older (around 7/8), their pants started fitting. Not all, but at least their sweatpants. I don't remember having great luck with Target and Circo was hit/miss.
It may not get better for a while. He's almost 13 and I recently bought a pair from Walmart that *looked* right, but are now hanging in his closet because they are too big. No more adjustable waist stuff at this age *sigh*
Good luck! I had to bite the bullet and start paying $5-10 for pants (hate that). These days, with my picky 14 year old daughter, I paid $15 for a pair of shorts *gasp*.