Although I cannot comment on what age is good to start someone young on shooting, I can comment on just introducing him to one of your handguns (Unloaded - of course) and show him that they aren't toys and just show him the in and outs of properly handling a gun. From all the stuff I've seen on the news, it seems curiosity is what gets the best of these kids that shoot themselves. Most haven't properly taught these kids how to handle a weapon and how serious they are if misused.
As it seems with most parents that tell their kids NOT to do it, the kids rebel and do it anyway. So if you advise him about guns and properly show him everything you can later gauge when he is ready to go shooting with you. Just a thought. Sorry for your loss of your friend, good luck on your new role as "Dad". I'm sure you'll do fine.
-- Beretta PX4 Storm 9mm --
-- Xbox Live Gamertag: Syst3m Ov3rrid3
-- Guns... It's never the problem... It's the SOLUTION