Sonny will be 16 next month. He has started to sit while he's drinking water. Is there a reason for this? Is he just lower in energy? He's never done this before, up until last year he preferred drinking out of the drip from the tub.
My immediate response would be that he just wants to. Cats can be odd and randomly decided to change their "normal".
However, given his age, I would think that some sort of joint or arthritis problem is setting in and it is hard to get into the tub to drink from his preferred spot. I would personally schedule for a complete exam to look specifically for arthritis but also anything else the vet might think would contribute to mobility issues.
An elevated water (and food) bowl is helpful for senior cats often with achy joints. My almost 16 year old cats just love their elevated ceramic food and water bowls from www.classycatdishes,com. Check them out!
I think this is not unusual for an elderly cat. My Speedy also will be 16 next month and in the last year or two I have found that she prefers drinking out of a tall pint beer glass, she sits up straight and just lowers her head a little. She also like to drink directly from the bathroom faucet, hind end on the counter and front feet in the sink. She will drink out of a bowl or her kitty fountain on the floor but it's definitely not her preferred method.
So the result is that I have several glasses of water around the house and I also have been trained that a loud meow means "turn on the faucet, please!"
Try putting the water dispenser up on a wooden block. Looks like it may be uncomfortable to bend the elbow. We use tall glasses for water-they love it. They won't drink out of bowls that low down anymore. Or use a stone slab from the landscaping area of your local Home Depot. If you don't have a hunk of wood around. Or use a small hardcovered book.