I'm pretty much in the same boat. I have a minimal left hand with no grip.
I dropped into this forum to post a thread about my eTap bike, saw this and figured I'd contribute.
I'm 63 and have been a serious cyclist since the 70s. I've ridden all my life.
A few thoughts: Sure he can ride. At his age the coaster brake idea is great. After that put the front brake on the right and don't worry about the rear. Really. It's worked for me for ever. I did add a cross brake lever to the right side for the rear just in case a cable broke, which has never happened, but I ride a lot of very steep mountains and it seemed prudent. I tried dual cable levers, but it never works as well as just a front brake (on a road bike)
On my mountain bike with flat bars I have a velcro strap I slip my hand through so I don't bounce off the bars. I don't find it necessary on a road bike.
I hope the technology I just wrote about on my blog trickles down to reasonable prices by the time your son is ready enjoy it. Read this what I just finished doing. Read it.
Curtis Corlew in Bicycle Land: SRAM eTap, Praxis 48-32 cranks and the perfect bike ?a review
And don't worry. He'll figure out how to do more than you imagine. I tie my shoes. I play guitar. I ride. I've played baseball. I play tennis three times a week. All with no left hand. And it's no big deal.
PM me if you'd like to chat on the phone about this.
p.s.
Positive Dad karma points to you.