There are a few songs we always do with him on stage (long train runnin' comes to mind) and we get a good reaction from the crowd. He isnt part of the band and doesnt ask for a cut of the money.
Your situation is exactly like ours, right down to the "Long Train Runnin'".
He doesn't ask for a cut of the money now, but let it go on and he will. Guaranteed. What happens when a person at the one of your gigs walks up to him and says, "you really are a killer harp player, and the rest of the band is great too, I want to hire you guys for a party next month" or same scenario but dude drops a $50 in the tip jar?
These are real examples that actually happened. Resulted in issues. Harp player saw himself as at least an equal partner in those transactions, some of the rest of the band (including me) did not. You never really know for sure if you are making that extra $ because of the harp or not. The longer you let this situation go on, the more the harp player will see himself as building up payment due to him. He wouldn't be human if he did not.
I will say that in our situation the harp player also can also play percussion (shaker) and since we gig semi-accoutically without a conventional drummer this helps too. But not enough to make me want to split the $ 5 ways instead of 4.....