My very skinny 7yo only wears his Under Armor pants. He never wears the other pants I buy him.

I've had good luck with Circo sweatpants. Some Old Navy sweatpants work but not the ones with the wider legs. Recently got a Cat & Jack pair with a functioning drawstring that aren't horrible .. but then an identical appearing set doesn't have a drawstring, which I missed so now have to return. I've also found Nike pants with the drawstring works ok.

just bought these for my skinny 4.5 yr old. And the waste fits very well in a size 4. For reference he wore 2T shorts all summer because everything else was way to big. So I;d definitely say they run very slim .

http://www.kohls.com/product/prd-2672623/boys-4-7-hurley-space-dyed-fleece-jogger-pants.jsp?color=Black%20Heather

American Apparel worked well for my skinny tall kids.

Adidas work perfectly for my 8yo skinny boy

Roll them at the waist. Been doing it for years with my DS who is in the 3rd% for weight.

Check The Children's Place, too.

I found the Garanimals sweatpants at Walmart to be cut very slim in the waist and they stay up well on my very skinny 3rd grader. And they are only $5 per pair! We got one in every color since they actually fit him!
I have also found that UnderArmor pants seem to run smaller in the waist than other athletic brands.
Good luck!

Jogger sweatpants are more fitted.

Circo are smaller than a normal size. (Target) My kid still wore size 6, but I had to get ride of the polyester sport pants early because they had a narrower waist.

Hanna Anderson - and they have a string to tighten if necessary. There are some very baggy ones I recommend the slimmer cut.

I had luck with addidas when my similarly built son was that age.

Ralph Lauren, Zara, and Jcrew are cut very slim.

Mine wear Adidas or Under Armour skinny soccer pants - try Adidas Tiro or UA Pennant.

My 7 year old wears the "slim" size at Land's end for regular school pants, but we need to get him some sweat or athletic pants for sports. We have been to Old Navy, Gap, and H&M and even the drawstring pants are literally falling off of him. Any other suggestions?