I'd keep him and feed him and fatten him up over the winter, and then release him next spring once the weather is warm. It's not his fault he was hatched so late in the season - we usually see baby snappers in the spring, but he's very vulnerable at this size. He's a snapper, so it's not like he's gonna "imprint" much on you like a duckling would - they seem to pretty much come with all instructions included.

I suppose, to be responsible, you could call the local Audubon Society ...