Urinating inside

We have an old cat 15 years he has started to urinate inside. It started off as infrequent and only at night but is now more often and now during the day. We lock in laundry area overnight cat flap access to outdoors. We do not want to banish him from the house in general. Are there any suggestions? We have had him examined by a vet and he appears healthy.
   
Thanks,
   
okitu56

Give the poor thing a litter box, so he can go inside without getting you all upset or getting locked in a room.
   
Maybe he's decided that he's too old to spend so much time outside.
   
Lots of cats use litter boxes.
   
It's also helpful for monitoring their health, as you can see the condition of the stool & the color of the urine.