No, he wasn't trying to be rude. It just means that this person doesn't use the Internet a lot, doesn't use email a lot, and doesn't know the customs. I personally believe that the idea that "all caps is like shouting" isn't even true. Yes, the custom is to use mixed-case. When people use all caps, other people tell them "all caps is like shouting" as a quick way of explaining to them why they shouldn't do it.

To most people, using all caps (all capital letters) is the equivalent of shouting, and yes, it's considered rude. In some cases, though, I think Americans use it when they're unsure of which letters should be capitalized. It's easier to capitalize all of them. It's fairly common, and I would not assume the person was being rude to you.

Sometimes caps lock does get stuck. Also, some people who have vision problems can read capital letters more easily. Sometimes shops write up their bills, inventories, and other things with all capitals too, so he may just leave it that way when he writes emails. Look at the words themselves to decide whether or not he seems angry with you, and don't just assume he is being rude.

It depends on what his reply to your email said. He could have been angry, but it could be that he prefers typing in all capitals or the "CAPS LOCK" key was pressed and he doesn't know how to turn it off.