I'm sorry you are going through this right now with your teen boy.
I thought I would make a few suggestions, even though my relationship with my teen is.. well.. I wouldn't say strained because we're very close, but difficult because she annoys me so much. I know I annoy her too.
It is my belief that teenagers need their parents just as much as they did when they were babes, if not more in some ways. I would NOT take his door off (when I was a teen, my friend's mom did that to her and it did nothing but further distance their relationship!!). It really sounds like your son is angry with you, for whatever reason, and probably for reasons he will either not admit to, or he is not aware of. You just had a baby (news to you, eh?) and that could be a big part of it.
Do you two ever get any time to be together without pressure or stress? I would ask him to do something with you. Hound him if you have to, to do stuff with you. Let him know you need him, even if it's help fixing something or making an art project, anything you can think of. I would also have him go out with you, to spend time together. I feel this is extremely important for the sake of your relationship. You have to take the baby with you of course, but otherwise, just you, him, and the babe. Maybe out to dinner, go shopping, hike leisurely down a trail, whatever it is that you two can stand doing together. I would talk to him about life casually and not pressure him to "fix" the relationship and the problems you're having with each other while you are doing things together. At first he may be very resistant to the idea, but I promise you it will make a difference.
In regards to that class, or whatever it is, I would tell him about it and how important it is to you that you guys learn to get along together again, and that YOU need it.
Does he have a cell phone? You can call him during the day just to shoot the $h*t. You can tell him you're ok with him going out after school, but please call you and let you know where he'll be. If he doesn't have a cell, maybe take him to get one, and let him know the rules for using it.
I'm usually pretty good at perceiving what's going on in a kid's head, and I really think he's angry with you. I do think you should try some GD techniques with him, and I've found that some of the same techniques you use with a toddler works with older chidren and adults too.
I really hope I haven't offended you by my suggestions, and I don't know what you have and haven't tried with him. Keep us updated will you? I'm really interested in what's happening with this.