I'm sorry but you should leave him and find someone who shows you the love and respect you deserve. Geoff.
Welcome to our forums.
I am so sorry to read what is going on with you. To answer your questions no, I don't think your partner has thought too much about your and your son and the pain it has inflicted. If he had thought about it then I don't think the situation would have occurred.
Even though you say 'why me' at least this is happening now before your son remembers it. My friend is going through a similar situation with her little girl and ex (who too has moved in with the person he was cheating with) and it is a horrible and sad time but it will get better and your son will remember the good mum he has.
Have you thought about seeing a counsellor to learn steps to move forward?