If he is showing a profit, and not incorporated, suggest that you pay him an additional amount of rent for the land AS IS.....standing crops that you're going to have to dispose of, messed-up waterways, gopher holes, etc............that rent would not be self employed income to him, whereas custom combining the corn for him would cost him 15% more when he sells the corn.

Make the rental agreement effective NOW..........pay him the cash when he wants it, although I would guess he would want it next year, due to selling a bunch of items now.

As to the amount............current market price x projected yield less factor for potential yield error (get his input on this)  minus trucking would be his and your gross take.

Establish two columns for gross take------his and yours.  From yours deduct your direct costs (diesel and hired help---- you already have the equipment, so depreciation and repairs and other such overhead would not be factors.) to arrive at your potential additional revenue.

As to his, try to project his costs of combining, drying, etc.-------but not for the field work and costs of input.  (His costs might vary from yours------apparently he has no combine, otherwise he would combine 50A)

Next--------factor in your emotions............how much do you want to pay him for his time, etc.  for raising the crop?

If I were him, I would factor trucking, drying, combining, ...................and hassle time.............with the net being the expected price.