Can an 18 year old boy who has not lived in parents home since age of 15, be forced to move back against his will?

Once someone is 18--even leaving aside the issue of whether he was previously emancipated--he is legally an adult. He may live wherever he wants, and no one, including his parents, can force him to move in with them. (This assumes that he is fully mentally compentent; if he is not competent, his guardian will determine where he lives.) Certainly, they can try to make it worth his while--free room and board, etc.--or could try using the "stick" not the "carrot" (e.g. threaten to disinherit him, if he doesn't do what they want) but they cannot force an 18-year-old to move in with them if he chooses not to.