Send a letter describing your boundaries and tell him you give him a license, revokable at any time you choose, to mow. You hope you get a response from him. Even if a denial it is your boundaries, you would have proof he had knowledge you gave him a license--thus eliminating the element of the possession being notorious. You might try mowing the disputed area yourself once in a while and getting a dated video of it to help prevent the "exclusive" element as well. I mean, if his only claim of possession is mowing and you are mowing too....
Also, I don't think one can get a prescriptive easement unless the other party is getting value from the possession. Doing mowing on certain areas does not seem to me to be value.