If the weather has gotten cooler and he gets big salads, he may just not need as much water. Or the water bottle might be imparting an "off" taste to the water. I notice mine drink more after I thoroughly clean their crocks with soap and water, even if they looked perfectly clean to me. I think bunnies drink more from crocks and they're easier to clean completely than bottles.
Two years ago, one of my bunnies wouldn't drink anything for 3-1/2 months. I wetted down all her salads and had to give subQ fluids about once a week.
When I posted for advice about her not drinking, I heard from someone whose neutered male bunny stopped drinking for 2 years. When he got him a girlfriend, the bunny started drinking again. After I heard that, I got a new male bunny from a shelter. After 3-1/2 months of not drinking, my girl started drinking again the day after their first bonding session. She never did go back to drinking water though -- only dilute apple juice.