There's not a lot you can do save for trying to come to some kind of agreement with him as to the terms, that or wait until it passes ownership and the new owners are willing to sell on better terms.
When you say clawback, do you mean with regard to not being able to use it for anything other than agricultural land or that if you get any planning on the land for development he can claim back part of the increase in value (overage provisions) or similar?
Are they clauses he has stipulated or could they be restrictions which already affect the title to the land which would have to stay with the land?
Can you come to some kind of agreement such as paying him a certain amount more upfront in order for him to forgo any such provisions?
Bear in mind that if the land has potential development value thenhe could decide to sell it to another party anyway though any rights you do have would have to be abided by.