No problem, I get bored here at work and I like helping people.
I'll have to preface this by saying I've never taken Calc with Matrices and I've never talked about it with anyone, so I can't say anything too specific about the class. Hopefully someone else can come into the thread and give you more info on that front. I can give you the general info you asked for on the Empo though, since I've taken three classes that at least tested there.
Yes, there is help available. There are students that are hired to help you (when you're going through lessons, they can't help you on tests, obviously) and there are professors there if the students can't help, although they usually know their stuff. I doubt your son's class will actually "meet," in that he'll be able to go over whatever time he likes during the week and do his lesson (although there is one manditory info session at the beginning). He will technically have a professor though, who he will be able to go and see if he has problems. Many professors have videos of what the lecture would be up on the math emporium site as well. The professor will go over that in the info session.
As far as the empo filling up goes, there is only one time that that is ever an issue, and it is never an issue for proctored testing. There are seperate sections for taking tests and for going over lessons/studying, and while the studying section MAY fill up around finals time, he should be done with lessons by then so it shouldn't be an issue. They will realocate computers to testing if they have to, and I've never seen testing computers full.
It stays very quiet, especially in the testing area. Even in the "regular" area it stays to about a library level. Sometimes the person next to you will be getting help on something from one of the students or professors and that can get a bit annoying, but that usually doesn't take that long. People generally want to get in and out as fast as they can, so they don't hang around and talk.