Please reconsider removing ANY live fish in the tank with him, especially because of his size. Fish will often pick on octos and annoy them to capture food scraps, causing stress. Fish are not a natural food. Even when they do eat them, we have seen that they only eat the soft underbelly and the dead fish become a pollution hazard if the carcass becomes lodged or hidden in the live rock. While live food (crustaceans like shrimp and small crabs) are excellent, you are providing an unnatural environment and need to consider compensating for the difference.
Most keepers find they can feed dead food at a regular time of day by offering it on a feeding stick (bamboo skewers work well as do nylon feeding sticks). This ensures that it is getting enough to eat and helps it become accustomed to you and the tank. You can try offering a piece of thawed table shrimp about the size of its eye. Once he starts taking that you can increase the size until he rejects it. I have found that most octopuses will also enjoy blue crab claws that I get from my local Asian market. I hunt up the loose claws in the live bins and then freeze any that do not smell. To feed, I thaw and crack the claws so that a good sized piece of meat is exposed. Interestingly, I always get the cleaned out shell left or pushed to the front of the tank, even when the food is consumed in the back.