I feel for you. The pain and stress of all this must be completely overwhelming. Is it outpatient or inpatient rehab? I guess what I'm asking is where will your son be while you are in treatment?
But I agree with GettinSober as well. What is normalacy anyway? Your ex being in a brand new relationship might be a little to distracted to tend your son's needs. Unfortunately, I do not feel qualified in any way to provide much here other than...bravo to you...for being brave.. for loving your son...for admitting defeat to the old way and beginning a brand new journey.
May you find peace within.