I adjusted my Compadre's lower shaft all the way in to the last hole and the total length measured 39" from bottom of coil to back of the arm cuff. About 28" from coil to the hand grip. Have your son stand with his arm at the position he would hold it and see if that will work.
The Compadre has a lot of holes to adjust in fact there really isn't any room to drill more. Plus if you did, I'm not sure if it would void the warranty or not. Hope this helps.