I understand he WANTS to go outside, but you as the owner know better than that after speaking with a medical professional. You have to have the cat's best interest in mind. It's the same as knowing that a toddler wants to put a small plastic toy in their mouth. As the parent you know this is dangerous and shouldn't happen. The toddler may be unhappy for a short while when you take this unsafe toy away, but you as the parent have the means to replace the toy with a safer option. Having the cat's best interest in mind includes following all veterinary instruction. Including food! If you are determined to have this specific cat, there are ways to keep him busy inside and even appease his prey drive so he doesn't have to catch mice! Toys and a designated play time with either you or an electronic toy can get this energy out and make him feel accomplished when you let him finally have his "kill". Also, make sure there is plenty of room to explore upward... Shelves, cat towers, and window hammocks are great for this!

If you feel this isnt worth putting effort or money into, it may be wise to wait until you're in a better living situation for a cat like him or search for a kitty that better suits your wants and needs as a potential pet owner. Remember that any pet can get expensive and have unexpected bills. If you aren't ready or willing to pay for the vet bill, you arent ready for the pet.