Here's a 2017 Haibike with 27.5" wheels and a 14" frame: (Link Removed - No Longer Exists)
They list the standover height as 29" though, so even though it is an "xs" adult bike, it is not quite what the tin says. If he is a confident rider he might get away with riding a somewhat big bike (I know I did when I was a kid!) Chances are you would just have to cut the seat post down to get it to lower enough to work.
The 24" wheel version would definitely fit him now, but as Chris wrote, he might outgrow it quickly.