Support to fiance with ADD

My fiance has ADD. He is loving, sweet and caring, yet impatient, impulsive and disorganized. I didnt understand him at first. But then we talked about ADD and how it caused him some problems at work before. He always tells me to be patient with him and that he promises to get better. My question is on how to give him the support he needs? And we have another issue: we have a communication problem. He tends to get aggressive and defensive eve if we're only chit chatting. I would like to avoid these tensions and help him focus more.

I need advice and some useful practical tips.