I like what you said here Bob.I like the handspan idea if his trouble is playing position. Or maybe move it one ball. Fist span?
If you want to increase ball spots, the onepocket.org website has a list of spots in order of strength: https://www.onepocket.org/one-pocket-handicaps/
There are lots of handicaps you could add or change to:
His fouls don't count.
Right after each break, he can take any two (or three) balls off the table for his.
If you don't already coach him some, twice during a game he can ask you for the best two ways to shoot a shot.
In the long run I think you want to steer away from spots that really change the game like handspan or "fouls don't count" as it's not really one pocket.
I've seen two friends play who liked to gamble for a little and the spot was 12-3.
My only addition would be on "fouls don't count" you have to be really explicit on what a foul and what an unsportsmanlike would be.