You can keep a room its house for a few months until it can go out the cat flap. In that room you can like make it a little homey for it. For example put comfortable clothes in a basket were it can sleep and stuff. Keep balls of yarn for it to play with. And that way it would stay busy all day. You can take him outside for a few minutes so it can take in some fresh air and so that would be good for its health too. But never let it go out alone or you might never see it again because cats don't come back to places which they are still not familiar with, at least I have experienced that.