Has he ever been tested for urinary crystals that could be blocking the urethra? This can cause some urinary spray.
You should also keep in mind that some cats experience severe arthritis so they will hesitate to climb fully into and out of the box because it's painful. You might want to try a box with lower sides. If you don't suspect this, a covered box will contain the spray and help you with this.
Let me know if you still have questions.
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