Adjust the scope, both the eye relief (probably moving it back toward his face) and focus. Use the lowest power on the scope which gives the widest field of view to find movement.
Another thought, if they have a bit of colorblindness (most common in boys) finding brown among browns can be tough. And lastly, it might just take a year or two for things to all fit. That is sometimes a reality of us being too gung ho with our youth. Don't sweat it. Do other things and enjoy it.
As for the recommendations to go iron sights...I know your axis doesn't have them (I have an axis) and purchasing a new gun probably isn't a practical solution for you. It seems some people have more money or more guns available to them than others.