Hello, XXXXXXXXX
This is called incontinence. There can be many reasons why it may happen. Urinary tract infections, an obstruction in the bladder, which is typically caused from stones, and weakening of the urethral muscle, which can just happen with age.
So getting him a vet check up to rule out things like stones and infection is important. If it is a weakened muscle there are medications to help it like phenylpropanolamine.
Is there a natural remedy for this? No, unfortunately there isn't. It needs to be treated medically. Just keep his bedding clean to prevent skin infection from the urine. Cleaning his bottom area with a baby wipe and using vaseline if the skin becomes irritated is a common daily task to do too if needed.
Sincerely, XXXXX
Expert in cat behavior and health
Expert in cat medical and behavior issues.