I started non-seriously dating about 2mo after I ended my 8yr relationship with BD. Mind you, in my mind, I'd wanted to leave him for about 6yrs, so emotionally I was pretty ready to move on. After 6mo I met a man I fell in love with, struggled with that for a month, then let myself just be in love. A year and a half after the split with BD I am engaged to the new man and we are having a baby.
One of the best words of advice I ever got when I was afraid was "Your son's relationship with his father is between them. There is nothing you can do about it. If they want a relationship, they'll have one." A year and a half later, my BD decided NOT to have a relationship with our son, and my son and his stepfather have a very strong loving bond. I don't know if he'll rebel in adulthood, but that will be between the two of them. It is not selfish to take care of your emotional needs, and it sounds like he and your child get along well too, so that's good Everything will work out exactly how it's supposed to work out. Best of luck!