Since Sri Lanka is male and not neutered, he will spray everywhere. It's just a fact of life that unneutered males spray. To compound matters, Dippy will smell it and want to cover it up with his own scent. It's a vicious cycle.
In order to fix the problem, neuter Sri Lanka. Clean every spot where they sprayed with an enzyme cleaner like Nature's Miracle. You'll need to get rid of the smell in order to stop the cycle. A good rule of thumb for multi-cat households is one litterbox for each cat and one extra. You'll need two more litterboxes for your house.