It doesn't have anything to do with it unless you count the fact that your cousin is so tired he pees in the bed cause he sleeps so hard from being so active when he's awake. Don't make fun of him just remind him to go to the bathroom before he goes to bed. Ask him if there's anything you can do to help him. My son is ADHD and he's 21 yrs old and in the Marines. He wet the bed until he was 11. It's not his fault and he'd stop if he could.
I think this is just coincidental, not causitive. There can be a variety of reasons. Here's a web site: nafc.org/bladder-bowel-health/bedwetting-2/bedwetting/ that will give you more information about pediatric nocturnal enuresis.