how do i let my 18 year old go even if i know hes headed for trouble..his girlfriend don't like us AT ALL

he has lived there before and he never hardly talked to us. they moved n a apt and things changed he started haveing something 2 do with us but they fought all the time. They r moveing back over there liveing n a rv n her parents back yard. its starting again he said it wouldn't but it has. I miss my 18 year old son we was always so close. what do i do???