Before my anxiety got to the way it is now, yes I internalized my anxiety. People I worked with and friends never knew. I even have some friends that now know say that they NEVER would have guessed, I was so calm.
I don't know of any way to really help him, except to be very supportive when he does tell you. When he isn't anxious, sit down and ask him what could you do to help him when he's anxious. It may be that he needs to be left alone, or he made need some validation, just listen to what he asks of you and try to be constant in supplying that need when he tells you he's anxious. Another thing is I know its hard to verbalize it, but maybe the both of you could come up with a cue he could give you when he starts getting anxious?
I know what it feels like to have anxiety, but not what it is like to support someone. I allow VERY few people to be around me or help me when I'm anxious, I prefer to be left alone under most circumstances. Anyways, I want to thank you for being supportive of your husband and wanting to help him!