However unreadonable he may be, when he comes on your property he is trespassing. You should confront him about this. If he were to be injured, he probably would not have a good case against you as a trespasser, but that doesn't mean he can't sue you and cause some inconvenience. Good luck.
The author of this post is licensed to practice law in the District of Columbia, Maryland, and Virginia. This post is intended as general information only, and is not provided as legal advice in connection with any specific case, and does not create an attorney-client relationship.
Putting up signs or putting locks on the gates would send the message pretty clearly. I would photograph the signs and/or locks and send them to your insurer ASAP to establish that you are specifically not permitting this behavior. Also, share your liability concerns and concerns that your dogs will get out with your neighbor. Maybe tell him that you don't want to have to worry about having to keep your yard safe just so he and his friends can trespass across there. Or offer to sell him an easement - that'll slow him down. Maybe tell him that your insurer advised that you would have to increase your liability coverage to account for him and unknown persons to regularly traffic on ATVs across your property. Ask your insurer for more tips on dealing with the neighbor. All of this should help to protect you in the event of a future liability claim.
In the meantime, are there any other ways for that guy to get to the field? How far down the street would they have to ride to make it across your yard? Can't they go around your fence instead of through it?
Probably not responsible. If you had an extremely dangerous condition (ex: deep hole hidden by grass) and you knew he tresspassed on other occasions, you could be liable. You need to contact your homeowners insurance to advise them of a possible claim. If you have medical payment coverage, it will pay for your neighbors medical bills regardless of fault.