How can I communicate better with my boyfriend who has anxiety and stress problems?

My boyfriend has anxiety and gets stressed out easily (by my standards), to the point where he can’t think straight or communicate with me. He doesn’t think he needs counseling or medication, but the problem is really taking a toll on us and causes us to fight fairly frequently. How can I stay with him? I am absolutely devoted to him but I’m starting to feel like my needs from the partnership are being outdone by the needs of his anxiety. I would say we are happy 99% of the time, but the remaining 1% is extremely brutal and hard for both of us, possibly because it contrasts so greatly from our normal day-to-day. I want to be supportive of him but at times I feel like I just want to scream “get a grip!!!!”