Around that age I started reading Harry Potter, and my son is now an HP fanatic. The only problem is that we got through them quickly, and I'm not entirely thrilled with reading past Book 5 to a first grader. Something to consider.
James and the Giant Peach would be great. When he's a bit older, try the Lemony Snicket series too.
Oh, and his friends are into a series called The 39 Clues. You could read those to him now, and then he could read them himself within a couple of years.