Yes, I did the same thing - even though H will have to cover my insurance for the next several years he does not get ANY of my medical info.
You should set up a health care proxy. If you become legally separated or divorced you will need one anyway.
You may consider making arrangements for somebody to care for your son if you do the all day treatment. It will be exhausting and you will need full focus on you. Perhaps your sister can help. I would not tell your son anything and let him think you are just going to work each day. Your son seems to report an awful lot to your H (and to you) so the less he knows, the better.
Stop talking to your H about anything other than your son and his needs NOW (not what he was like when he was a newborn). Your H is just using passive aggressive tactics to flaunt his and OW's newborn in your face.
Mystik - you will look back on this day and soon see you did an AMAZING thing for yourself. For real, this is just the best news I have heard from you since you started posting. From my own experience when I started getting the proper treatment I was scared to death but the sense of relief was nothing short of one of the best feelings I ever had in my life.