My son (12 years old) is skinny but athletic, I gave him 10-12 and they are a bit loose.
He can still wear them and grow into them, but the underwear needs to be a bit tighter.
Arctix Youth Pants Reinforced: Large Boys' Clothing for Active Adventures
46 Review
Waterproof Cozy-Dry Rain/Snow Pants for π§ Kids by JAN & JUL, with Fleece Lining
64 Review
π₯ Stay Warm and Active with Rocky Fleece Thermal Underwear Bottoms for Boys
44 Review
π Boys' Outdoor Climbing Convertible Trousers - Waterproof | Clothing
47 Review