I would go for a ten hole diatonic probably in C. I'd go for a plastic comb, maybe a Special 20.
Granted, as John MacKenzie mentioned, there is not a complete scale in the lower register, but this will be most useful as he learns to use it. Few harp players read music to the harp. You can't see the numbers on the harp while you are playing it, so it's mostly an ear instrument.
I and my brothers and sister started on Marine Bands, Christmas gifts from our parents or Santa, which were the only 10 hole diatonics available at our local music store in the late 50s/early 60s. I gave my sons/nephews/nieces Hohner Blues Band harps when they were very young, not a good quality harp, but not a big investment either.