You might try the same question in the Recreational & Family forum.
He's going to be a tough fit, is my guess. My inseam is 26", and the smallest adult frame more or less works, but I'm 5'6. He might be needing something with a very short top tube.
Kids bikes are almost criminal, by the way. My neighbor's boy has a kid's Specialized Hard Rock, and the damn thing feels like it weighs as much as my Giant Cypress. I know, you can't build a $750.00 bike and sell it for $200.00, but still. . . .
Some people are like a Slinky ... not really good for anything, but you still can't help but smile when you shove them down the stairs.