According to the company that sells swimwear on our swim team, you need to measure the boy's waist to determine the size of the swimsuit to order.
I think we ordered 28 for my six year old who is a little heavy (husky clothes are too big but regular clothes are a little tight). It has a durable feel and the bright color makes it easy to find in a bag.
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
Arctix Youth Pants Reinforced: Large Boys' Clothing for Active Adventures
46 Review
Boys' Adidas Active Sports Tricot Athletic Clothing
45 Review