My cat is constipated and won't eat his Rx food. What can I give?

There's not perfect answer there, but if he's not really eating the prescription food anyways AND the food would only help if he was drinking enough water to have dilute urine, which I'm guessing he isn't because he's constipated, then changing foods is unlikely to cause problems. With what you're describing, he could even need subcutaneous fluids at home to get enough water in him. I'm assuming he's had general bloodwork done to try to figure out what's going on, because I would be concerned about some underlying organ dysfunction being involved here. If blood work is normal, I'd experiment with diets, water fountains, and even the fluids under the skin to see if any of that helps. Wishing you and Flash the best of luck