For me, it would have been pretty quick after my son got the job. But, that isn’t the case here and not every situation is the same either.
However, you have granted him continued arrangements and now it is time to do the hard thing, and that is to have a sit down conversation and tell him what goes where.
He will probably get “mad”, but it’s your house. You raised him, fed him, gave him clothes, care, and love, and now give him the time to find a place, but not over a certain date.
If he doesn’t find or says there isn’t a place he can get, print some out or circle the newspaper listings and give them to him. Review this frequently, but not by smothering him. After the deadline, talk about it constantly, and add some guilt about how old he is, his new upcoming family and wouldn’t he rather bring them along his way, etc., along with other little tidbits he probably doesn’t want to hear. Stand your ground and don’t flinch. He knows he has had it easy and has been taking advantage of you.
And speaking from a personal frame of reference, for the long term and depending on the personality type of some adult children, they may need additional emotional and parental support even after they move. Keep in touch often, see how things are going, befriend his girlfriend, talk about his job, how the pregnancy is going, send/take them some baby things, etc., and try to keep communication with them ongoing. In time they come around if they have left “mad”, and it gives one a proud moment when they finally say, “You were right” about that and many other things. :)