Everybody else here has already addressed talking to your grandma first. I agree, not a lot you can do when the landowner has other ideas.
My only suggestion could backfire. How close are you to your cousin? Maybe you could talk to him or her directly and explain the situation, what you are trying to accomplishand get that opinion. The reason I say this may be risky, is if you are not close to your cousin or there is family squabble going on, then he may rush right to your grandmother and try to further provoke the situation.
250 is a good chunk of land. If you can only take 1 - 2 bucks by your state regulations, thenI believe your efforts will still be rewarded. You may have to bite your tongue a little bit if he kills a smaller deer, but be happy for him even if you have to fake it. That way he will see howhardwork and patience pays off when you bring in your record book buck. If you have talked to the neighbors and know the land,then you also know where the big deer are going to be. You should also know where your cousin is going to be hunting. Big deer do not get that way being dumb. When he blasts the fork horn, he might push the big boy right to you.