I'm guessing this is a break barrel? If it is a break barrel, has the barrel ever gotten away from you or your son when cocking the gun or has the gun been fired while the barrel was still open? If so, then there's a good chance that the barrel has been bent upwards. Many break barrels suffer from barrel droop which makes them shoot low. But if yours is shooting an inch high at only 15yds with the scope adjustment maxed out then I would check the barrel to see if it's bent.
See if you can lay a straight edge on top of the barrel block and check the gap between the straight edge and barrel. You may need to remove the rear sight (if you haven't already) to do that. If the gap is narrower at the muzzle than it is in front of the barrel block then your barrel is bent up and needs to be straightened.
"You've come far, pilgrim."-------"Feels like far."
"Were it worth the trouble?"-----"Ah, what trouble?"