Based on the Answers section on the Amazon page I ordered a large size for a size 10 boy. He's really big. I had to order medium but followed the seller's replies. I would say order the same size or smaller depending on how you want it to fit.
The Good Day Lab Clean Shield: Ultimate Comfort and Breathability in Tagless 4-Way Stretch Polo
43 Review
π Hanes Boys' Short Sleeve T-shirt Value Pack (3-pack) - Essential Wardrobe Basics
44 Review
Star Wars, Marvel, and Disney Short-Sleeve T-Shirts for Boys from Amazon Essentials
50 Review
Boys' Long-Sleeve Disney Star Wars Marvel T-Shirts by Spotted Zebra
46 Review
π₯ Stay Warm and Active with Rocky Fleece Thermal Underwear Bottoms for Boys
44 Review
Waterproof Cozy-Dry Rain/Snow Pants for π§ Kids by JAN & JUL, with Fleece Lining
64 Review
Funny Guy Mugs Tearaway Sweatpants: Playful Boys' Clothing for Fun and Active Days
49 Review
π Boys' Outdoor Climbing Convertible Trousers - Waterproof | Clothing
47 Review