For an 11 year old kid, I think the FM3a might be the wrong camera. It's more of a collector's item than a bruise-around kind of camera. Probably a Nikon EL2 or FE would be a better choice in that respect. Something cheap and easily replaceable if it gets dropped too many times. Pair it with a 50/1.8 Series E, and it's almost as small as the FM3a/45 kit, but with a much cheaper and faster lens.
That said, if the OP's camera can be verified to have a working meter, it'd be the best kind of camera for Richard's son. Completely manual, so he can learn how to properly expose film, and fairly bulletproof. My first Nikon camera was a Nikomat FTn. Got it when I was 11.5 years old. When I sold it 10 years later, there was no corner that was angular. Every corner had been rounded due to being dropped and bumped into things. But, it still shot perfectly. It also didn't lose much value, since it was still a relatively common item (as it is today). The same could not be said about an FM3a. It might take the abuse, but, it'd quickly lose about 50% of its value by being damaged. And, parts aren't as common for it, since so few were made.