Select a mattress that fits the size of your child's bed. If you're not sure of the size, measure it in inches and compare that measurement to the mattresses you're interested in. Most beds for children will fit either a twin, twin-long, full, full-long or three-quarter mattress.
Bring your child to the store to let him test out the mattresses. Every person has a different preference. Only the person using the mattress can tell you which is most comfortable.
Look at your child as he lies on the mattress. The mattress should adequately support the shoulders, hips and lower back while keeping the spine in a natural shape, which is the same shape as when you're standing. Make sure these areas aren't simply resting on top of the mattress, meaning it's too firm, or sinking into the mattress in an unnatural way, meaning it's too soft.
Talk with your child about the mattresses he tested. Be sure to have a discussion after each mattress because children can forget quickly. Since your child will be the one using the mattress, his comfort and preference are most important.