I bought a set for each boy. I bought a 6 year old 6-7, an 8 year old 7-8 as they always run small. He has gained some weight so he is a bit stockier and no larger size is required for his age. Tried 7-8 on it and it's tight. Worn by a 6 year old, fits perfectly. It's stretchy so it might fit an 8 year old but it was uncomfortable enough for him that it hasn't even been washed. I don't even want to see how much denser it can get after it's gone through the dryer. So you should increase the size.
Van Heusen Sleeve Dress Poplin
42 Review
π Hanes Boys' Short Sleeve T-shirt Value Pack (3-pack) - Essential Wardrobe Basics
44 Review
Boys' Long-Sleeve Disney Star Wars Marvel T-Shirts by Spotted Zebra
46 Review
Star Wars, Marvel, and Disney Short-Sleeve T-Shirts for Boys from Amazon Essentials
50 Review
Boys' Adidas Active Sports Tricot Athletic Clothing
45 Review
π Fruit of the Loom 6 Pair Cushion Ankle Boys' Clothing: Comfy and Stylish Selection for Active Boys
57 Review
π Bold Pack Sleeve Active Hoodie Sweatshirt Set for Boys' Clothing - Stylish and Comfortable!
47 Review
Arctix Youth Pants Reinforced: Large Boys' Clothing for Active Adventures
46 Review