Cat Pooping in Bathtub and Litter Box?

My 11 year old son and I are doing everything we can think of to help our cat, Grayson. Grayson is an 11 year old male cat and has been blind since he was about 6 years old. Between my son and I, he gets plenty of love and attention. Grayson has started pooping both in the bathtub and in the litter box in the past few months. The cat litter box is about twice the size that he is and has always been located in the bathroom, near the bathtub. He has always pooped and peed in his litter box without fail. I would move the litter box as suggested, but figured that's not really an option considering he is blind. Plus we live in a small house and there really isn't room for a litter box anywhere else. We tried switching up his litter to a previous litter he has used, but this isn't working. I'm kind of nervous about adding a little water to the tub because of his blindness. His poop looks normal. Any ideas on what to do?