Welcome to the board!
It sounds to me like your son is not done using. This is HIS recovery and unfortunately for the loved ones/family members of addicts, we have to understand that it really doesn't matter what we say, how long we try to convince them otherwise, or try to reason with them...if the addict is not done using, no amount of time spent in detox, inpatient treatment, or rehab will make much of a difference. If your son doesn't surrender to treatment, doesn't admit to himself that he is powerless over his addiction and needs help and needs a program to stay clean, he could stay there for the full 28 days and come home and use.
Get yourself to an alanon or naranon meeting and get support. You're going to need it.
Best of luck to you!
Keep posting, we're all here to help.