Write up a contract that says something like
T=You will initiall re-organize contents of the arena to his approval, which will provide you with 1/2 the space for your horses while also providing eb=nough room/organization for him to access the stuff he has store there.
Also - you will "groom" (flatten out the bumps) the arena minimally once per week.
In other words exchnge of work on your part (no work on his) in exchange for use of the arena - this would be a WIN-WIN situation and he should readily agree unless you fail to uphold your end of the bargin.
Document the agreement (how long you could use it, when you can start using it - based on accomplishing several things he wants first, etc.) and when he reads it, if he agrees, both of you sign it and both of you get a copy. Note that if you violate the agreement he has the right to take back the arena - and all your work will be for naught.
Dressage is for Trainers!