It's possible that his property line ends at the eastern bank for which he can not post since it is not his property beyond the bounds of the river itself--unlikely since property dividers rarely end on the variable banks of river, rather they occur in the middle which is relatively constant.
There are some property owners that will dishonestly extend their reach just because they don't want people fishing near their property. This happens all the time and the DEC and the town, village clerk can help with issues like this. All it takes is a call or a stop by. It's part of the clerks job to handle property issues, you wouldn't be the first to ask something like this. If the river is navigable, anchor a boat, canoe or raft just offshore and you are allowed to traverse private property.
More information can be found here: http://www.nyfalls.com/trespassing.html