ADD is ripping our blended family apart

We're a blended family. My partner with ADD and his two girls (12 and 17) and me and my son (14) and daughter (16) moved in together about 6 months ago. The children get on really well, we all do. Lately though my partner has been very strict on my son - and is driving my son crazy - he has very high expectations of my son and my son can't do eneough to please him. My son up until now has done everything he has been able to to be understanding about my partner's ADD, but he has had enough and wants to move out. They had a big argument the other night over some ridiculous thing that my partner misunderstood and he said very mean and nasty things to my son. I wasn't there at the time. So, my son wants to leave, I can't let him go on his own, if he goes I go....a deal breakerl...the family is over. My ADD-partner won't budge, he says it's going to be his way, one way or the other (whatever the hell that means). Have tried talking to my partner but i just get the silent treatment when it comes to my son. He's backing me into a corner - of which there is only one way out.

Can anyone offer assistance, maybe in a similar situation?