me being in a wheelchair i have thought of the same ideas as you. fortunately i've still got the use of my arms. no abs tho. so stability is so so. thats the most important thing when it comes to me shooting something bigger then my ar's. there are two things that come to mind.
one would be some sort of rifle rest that would go on a table or tailgate that can be wheeled up under. either sideways or straight from the back. if he can lean into it and get the butt of the gun on his shoulder to compensate the kick and his body weight wanting to fall forward, it will be comfy to shoot. something he can put what arms he has up on. he'll know what i mean. stability is the issue.
two would be something he could roll into that holds the weapon for him and allows his back to rest firmly on the backrest of his chair so he won't go anywhere. again its the stability that is an issue. i'm sure his chair is custom fitted to him and gives him alot of structure and support.
when i'm shooting freehand from the wheelchair i hug my gun and shoot towards the left. it keeps my back firm against the back of my chair and gives me the ability to aim without being unsafe (being that the guns weight will tilt me forward)
with that being said, if you have a 22 lightweight rifle, like the m&p 15-22 or like wise, it will help get the feel and prepare him for a heavier weapon. i keep my vert grip as close as i can to the magwell so i can hug and grab the gun as much as possible. also, if the weapon is front heavy it will throw off his balance. keep the weight to the rear of the gun to help his back support.
if he has a hard time gripping the gun, glue gloves onto the grips so the gun won't come loose. i've seen that help others.
i hope this helps. if anything i can do, or questions, feel free to PM me. good luck and thank you for helping him out. that means alot to me to see this.