I really do not know if I can help but I do feel for you both. Sometimes submissives can be exceedingly frustrating. (can't we all be)
It is very hard but dominants must have control over themselves. If you cannot control your emotions, how can you control another person?
I am not saying that it is totally wrong for him to get frustrated or angry, but he does need to figure out why it is happening and both of you work to avoid hitting those buttons if possible.
He is right, even if he steps out of the dom role, when this happens you must stay submissive. Being a dom or a submissive is not always easy.

Just a total guess here but his fustration might be from him not knowing or not feeling like he is dominant enough for you. That he isn't doing it right and he doesn't know how to correct it.

Tell him to come to the chat room sometime, maybe we can have a doms night and just talk about things like that in one of the side rooms.

And remember, being a submissive isn't just about chains and rope and whips, it is about giving yourself to your dom/master completely. He is taking a lot of responsibility, you have to show him you trust him to use it.

good luck!