Seconding the recommendation for primary.com. It's that or adjustable waists.

primary.com. My kid is 7% in weight and 94% in height.

You could also look in the girls section for pants. They are always cut slimmer than boys stuff. I have the opposite problem. I have a girl and we need the boxier boys cut for pants.

Zara or jcrew skinny or gap slim

Old Navy has slim sizes in certain styles and they are slimmer than the slim sizes at Lands’ End. You have to order them online because they don’t carry them in the store. We did a lot of the slim sizes with adjustable waists in the slim/skinny fits.

I've had good luck with Gap but it has to be the slim style. They do run long, too.

Others who make slim: Lands end (great for swim suits)
Abercrombie - great for jeans and chinos for a very slim teen.

My son has always been very skinny and I needed to take in his pants for many years. The slim pants.

Old navy rockstar pants. Skinny and long.

You need pants with adjustable waists, that are cut slim or skinny. My son's build is similar and we get pants at H&M for him.

H & M

Cat & Jack pants are skinny and have adjustable waists. My skinny kids wear those.

+1 Children's Place, the Slim sizing.

Btdt. Children's Place cargo pants with adjustable waist.

https://www.childrensplace.com/us/p/Boys-Uniform-Woven-Pull-On-Cargo-Pants-4-Pack-3019986-BQ


They have "wind pants" in size slim, too.

My rising first grade son is SUPER skinny (literally second percentile BMI for his age). Ped isn’t worried and he eats a ton, so this is just about keeping pants on him.

He doesn’t like jeans much but I need something that will stay up but still be long enough.