Declining admission on CSU portals after accepting to one

Does the ability to decline admission go away after you have accepted admission to another school in the CSU system? I just wanted to be sure my son did it - but can’t figure out how for CSUF, SJSU (these two he couldn’t find where to decline even before he accepted at his preferred campus) and Cal Poly Pomona (maybe since he acceptedit admission elsewhere? Do the systems talk?). Why do they make it so difficult to decline? Would appreciate some insight! Thanks.