Natalie, have you checked him for fleas. Going after them could cause the hairballs.
If you reduced his food that would explain weight loss... Is his appetite okay? Does what goes in come out normally? Some times what seems like weight loss in older critters can be loss of muscle tone.
Like Nammu said, watery eyes could be allergies. My guys who have allergies struggled this summer.
I agree with Nammu. It sounds like you are taking good care of him. Seems like he is an old man with old man stuff.