Family Law Questions? Ask a Family Lawyer Online.
Unfortunately, Father and Mother need to come to an agreement.
Once they do, it is relatively simple, adult child is treated as a tenant, they give adult child 30 days notice to vacate, if he does not leave they file an "unlawful detainer" (eviction) action against him and get a possession judgment against him, the sheriff will then enforce this. (Adult child does not want this to occur as having an unlawful detainer filed against him will make it much harder for him to get his own place, so most will vacate voluntarily).
If father and mother cannot come to a resolution between one another, counseling or mediation may be worthwhile (a third party mediator may help to provide perspective and scope to the discussion so that a resolution can be reached).