There is a lot of information about rewiring a rapidstrike as well as controlling one with an arduino. If you already have an electronic switch that his son can easily activate, you just need to rewire it, as long as the switch is rated for whatever current the RS is operating at. Using an arduino to control the motors would allow you to connect more elaborate sensors for activation, like EMG (electromyography) sensors, which are used to sense muscles tensing just like in a medical grade prosthesis.
I'm not sure how he would aim, but you mentioned something about his armpit, so I'm guessing he has something there if not fully formed arms. Depending on that, a properly adjusted shoulder strap might be enough to get the job done. Swapping out magazines and reloading could be a challenge, but then I've seen kids born without arms do some amazing things with their feet, so who knows. What I do know is that one of the best Nerfers I've every played with was born with one arm and that really hasn't given him any trouble. If that sample size of 1 is any indication, this kid is going to be a beast on the field.
Edit: There's also the Rival Zeus, which uses balls instead of darts. It's easier to reload and there are modifications to extend the magazine to hold a practically infinite amount of ammo. The only issue I can think of is that while it's electric like the rapidstrike, I believe the trigger is still mechanical. I haven't used one myself, so I'm not sure what it would take to get one on a remote trigger.
Edited by Langley, 26 December 2016 - 02:03 AM.
