I'll suggest that Theseus might be a better fit for D&D than Heracles.
Theseus' six labours involved ogre-men, each of whom could be the focus of a session (Sinis with the trees, Scyron with the cliff, Procrustes with the bed). They represent a lower-level challenge than going to the underworld and travelling to the ends of the earth. Also -- your son doesn't need to know the stories, and may find different ways to defeat them!
Going to Crete is also interesting. Theseus originally volunteered to go as part of a tribute (7 boys, 7 girls) to feed the minotaur. It's a chance for him to step up as a hero, but he might not do it right away. That's okay! (It might be interesting if you planted the seed -- "It's tribute time and several young Athenians have been chosen for this year's sacrifice" -- and then offer it again after a few labours, and then again, until he decides to try to do something about it.)
Theseus does have an underworld story, one in which he is supporting a friend (Pirithous). Leading up to that might be a good campaign.