Right, because kids shouldn't handle snakes...
(A child at an event dealing with my 6'3" guyana boa just fine, he was respectful, gentle, and calm; he loved Bear.)
I've seen plenty of kids this age, younger, or older, handle snakes JUST fine without then dropping dead. I also know plenty of friends, breeders, enthusiasts with kids of all ages handle snakes, sometimes better than the parent and they're all just fine. Some people grow up around that and so they know how to handle them. Now I wouldn't suggest a 4 yr old handling a hot herp.... But that's just suggested for anyone, at least not without EXTREME precaution and care.
I say, good on ya for letting your son be exposed to herps. I understand that it just isn't for everyone, some people have true phobias and can't get over it no matter what, but exposed and education is key to understanding the mysterious and beautiful.