Sadly, people ignore “no trespassing” signs for the most part. 1 single strand of hot electric fence with " no trespassing" signs hanging from it might do the trick.
Our property borders along a big creek just off the county road. People think nothing of blocking our driveway ( to park) or driving through our planted row crops to fish at the creek at times.
When this happens we do find them and tell them we prefer they not. So far it works.
Personally , as awkward as it may be, I would just go over, welcome them to the neighborhood and ask that they not to ride through your fields to get to the road, if you don’t want them to.
I would go over and talk with the older gentleman and make sure he stays on the perimeter of the field but I would continue to let him ride. It may be that he had an agreement with previous owners and I think he may be more contentious about ground conditions. Have him sign a waiver .
When we lived in MN I had only our 10 acre hayfield to ride on or the paved busy highway. The person who owned the 100’s of acres of crop land adjoining our small 17 acre property let me ride on it after I asked him.
I was grateful and tried to do my best not to damage a thing.
I ride through our hay ground all the time except when the grass is taller and/ or ready to cut. When the grass is short or dormant it does no damage as long as the ground is not too wet or muddy.