If son is 21 and thinks he's old enough so he can make his own rules, fine; in his own space! Tell him to start looking for where to move (don't have to "kick him out", just nudge him out) as he's "too old" to be living at home, there's not enough space for him and girlfriends and use your husband's/daughter's and your own needs for study quiet and taking care of selves; only those taking care of selves need apply to live in this house.