Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about Mr Stobbs' habit of urinating over the sides of his box. I can completely understand your frustration with him as cat urine can be quite pungent and hard to clean up.
If this is a new behavior for him perhaps he is having trouble getting into a squatting position to urinate and finds it easier and more comfortable to urinate the way he is now. If that's the case using something to control arthritic pain/inflexibility may help.
Long term for joint pain I do recommend using a combination of a
glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an
omega 3 fatty acid (like 3V Caps or Derm Caps). These work synergistically and
improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some cats do very well with them alone. They are available over the counter.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_feline.htm
If that's not enough he should see his veterinarian for prescription drugs that are more potent. Veterinary drugs we can add include a nonsteroidal like Metacam (Meloxicam), or Onsior (robenacoxib) .
Or has this always been a habit for him? Some male cats like to "mark" their litter box by urinating on vertical surfaces. If that is the case with him using a closed in top only access or more closed in box may help. Here are some examples:
I hope that this helps with his behavior, please let me know if you have any further questions.