As a general principle, the longer the status quo goes on, the more difficult it is going to be for the farmer to argue that the earlier regime is valid. He does have a legal right of access, as you acknowledge. I wonder whether the old regime does actually specify in words that the track is 15 feet wide? - if it does that is the width. if it is 20 feet now (and you have allowed it to 'grow' over the years, then eventually it will be become a 20 feet wide track in law.
The legislation by which these sort of arrangements happen (in England & Wales) is by the Prescription Act 1832. Google it for more info. The gist is that he would need a minimum of 20 years access without permission, in secret, reasonably continuously, after which he could claim a private right of way. You farmer already has such an access agreed - your issue seems to be whether that access could shift over time. On that specific point, I am not sure.