What is the issues? Do you have a cat door or something that might allow him to get outside? Otherwise, just don't let him out. Guess I'm not understanding. If he used to go outside and now you don't want him to, just don't let him. Eventually, he should get used to it, although you'll have to keep an eye on him to make sure he doesn't "sneak out" when the door is open to let people in or out.
We have an indoor cat who used to be an indoor/outdoor cat, and still, after 8 years of being an indoor only cat, he tries to sneak outside once in awhile. We just know his tricks and try to foil his attempts by putting our legs in front of him (or hold him) whenever we open the door.