I think the Hobie 18 is a great beginner boat, because as mentioned above, it doesn't pitchpole like a 16. If every 8yr. kid in an Opti can figure out the boards, I expect any adult can too.
The only real drawback to the boat is raising the mast, but he's going to have to learn the proper technique sometime, and as soon as he does, no problem. And it has a rolling jib which, if the wind comes up and you are new and overpowered, is a nice option to have.

My first cat was a Hobie 18, I sailed it solo and with 2-3 kids on board, the 16 is harder to sail well and will pitch you off if you make a mistake, the 18 is more forgiving so therefore a better beginner boat.