Not sure you should approach this as self board. You are using his land. Look around and see what other people charge for renting an acre or two. You said you are paying for the tractor's gas (thou that is a lot of gas to move hay if it is always $10) so it sounds like it is just a land lease.
I am in southern California and I pay $60.00 a month. It covers the 24X48 corral with 8X24 shelter, automatic water, use of the arena and round pen, insurance and dumpster for manure removal. I buy my own hay, feed her and clean the corral. No tractor, she provides the golf cart and muck buckets to clean the corrals. We are responsible to replace anything we break, but they do all of the maintenance.
After reading all these listings, sounds like a have a really good deal.