What can I do about my controlling partner?

My situation is difficult. We were married before and I had had an affair and we divorced (not going into details). I realized how stupid I had been and all the horrible mistakes I had made and asked for forgiveness and said I would be willing to do whatever it takes to make this relationship work. Well, he moved me back in a few months ago. We have JOINT custody of our son but he is the primary PHYSICAL, not legal, custodian of him. Since I have moved back in however, he thinks that we should only discipline our son the way HE wants to do it. For instance, he put our son in bed for the night (he is only two) and then came downstairs. Our son started to whine/cry because he didn't want to go to bed. My partner got up and I said "please just leave him alone, he will fall asleep on his own"...well he went upstairs anyways and started threatening to take his nightlight away if he didn't go to sleep and our son started to cry, so then he decides to shut his door (which our son hates because he is afraid of the dark) and let him SCREAM. Mind you this whole time our son was staying put in his bed but just whining. I went upstairs and asked what he was doing and he told me not to open the door because he was trying to "teach him" but I just ignored him and opened it anyways to comfort our son. Then he gets extremely angry at me and tells me he's tired of my **** and says I am undermining him. I feel that he was being ridiculous and that I was not undermining him because he is not our son's only parent and I don't feel he realizes this. He reacts this way about everything with the attitude that it's his way or no way. How can I deal with this?