Mate in 2 from my son [pretty hard]
It's a good start! In case your son is interested in becoming a problemist you may point out to him that efficient use of material is of the essence. In his diagram the white king is workless. It could be put to good use by placing it on g4 instead of the pawn. The solution still works fine!
I pointed that out to him earlier and he said he just liked having the pawn there better. No accounting for 9-year old tastes. 
Fantastic work for a nine year old. Great imagination and economy of means - even considering the pawn on g4.
The moves and areas covered by the long-range pieces definitely stir up his imagination - a very promising circumstance for his further chess development.
A nine year old with a 1969 USCF??? I seriously should just quit chess.
i know him. He's 12 and like 2400 now.