Awww. I'm so sorry for you and your kitty. It is a myth that cats are happier outside than inside. It's much safer for a cat to live inside, and it greatly extends their life.
You need to do whatever it takes to get your cat back inside, even if it means tricking or trapping him. You can get a safe cat trap and put the food inside the cage. It will close once the cat is inside. This is the fastest way to make sure you get him inside as soon as possible.
If you do not want to do this, you can try another method that will take a little longer. Start putting his food and water dishes immediately in front of your door (but still outside). Repeat for a few days. Then, keep the same placement of the dishes, but keep your door open while he eats. Be careful to keep the appropriate distance from the cat so he feels comfortable eating in front of you. After he's done eating, shut the door and let him continue being outside. Repeat this for a few days. Then, move the food dish just inside the doorway. Again, let him leave when he's done and shut the door. Repeat for a few days. Slowly move the food bowl further inside, always leaving your door open so your kitty doesn't feel trapped and get spooked. If this happens, you may need to start the process over again.
Eventually, you can get the food dish far enough inside that you can close your door and trap him inside. He may hide. Be patient. Talk to him. Don't try to remove him from his hiding place. Over time, when he feels comfortable, he will come out.
Since your cat now knows there is an outside, you will need to make your house very fun for your cat. You will need a couple tall cat trees with different levels. The trees need to be at least 6 feet tall. This is quite a bit of money, but it will be worth it in the end. You can also put shelves up on the walls, like stair steps for him to climb. Set up lots of things for him to climb and toys to play with. He won't interact with them right away, but again be patient and he will. Try getting a pole with a feather on the other end. Use this during play sessions. If he likes it and plays with it, this will help you bond. Be sure not to leave the toy out. It only comes out when you are playing with him.
If you have any problems with him using the litter box, you may have to try combining the litter with dirt from outside or some of the sandy types of litter.
If you do not have him neutered, you will need to do so right away. This will also decrease wanting to go outside.
Lastly, don't let him go outside for any reason, even when begging loudly. Outside no longer exists.
He should begin to bond with you again. Don't try to rush the process. Let him drive the speed of interactions.
I wish you and your kitty the best. I hope you will eventually get your loving cat back. Let us know how it goes.