Oh goodness, your kitty really lucked out to find someone willing to tolerate long-term spraying.
Honestly, I don't know anything about kitties and Prozac so hope someone else will have suggestions for you. If he got used to the 5 mg, maybe he'll adjust to the larger dose? Or is it possible to increase it to 7.5 rather than 10?
I assume you've had him fully checked out for any medical issues. Have you tried any anti-anxiety remedies, like Rescue Remedy, or HomeoPet's Anxiety Relief, a pheromone diffuser, or a calming collar? Those might help with stress due to the changes in the household and allow you to decrease the dose as well.
Also, you're using an enzyme cleaner like Nature's Miracle to clean up wherever he's sprayed?