child 10 constant erect penis

That might be one of the sighs of premature puberty. He is about to enter puberty anyway in a year or two, but, since we all are different, some kids start it earlier and others get all the puberty-related changes later. There really is a phase in puberty during which boys have erections at most embarrassing an unusual moments, like in the middle of the school class without no reason at all. I think if you even phone your family physician that you will heat the same story. The only thing that is important here is to realize that all this is completely normal just a phase of growing. But call someone you trust, like your doctor, and ask if you need additional reassurance. This is one of those things every man who read your posts can confirm it.