You've gotten a lot of good advice already, but I want to second the use of the 1,2,3 method - say what the consequence will be (if i have to count to three, then no tv today), then just count (hold up your fingers as well), have some pauses in between, then when you get to three, follow through.
There are books to read on the topic (I've heard 123 Magic is good), but I just made up my own system. It's important to hold up your fingers, because thankfully you can get to the point where you just have to catch their eye and hold up the fingers (1, 2...) and not say a word! Has been working wonders for my oldest since she was 3 1/2 - can communicate with her from across a room, and silently in a church pew or at a meeting. (She's 5 now, so doesn't even need any consequences spelled out, she knows she won't like them.)
I've just started this week teaching this to my youngest who is 18 months - she was carrying her cup of milk around instead of leaving it on the table. She's allowed to drink from it, but it has to stay on the table, not travel around the room. Did the 1,2,3 thing, then had to go get the cup and put it on the table. She cried and followed me back to the table, but then I soothed her and she had some more to drink. She tried to carry off her cup this morning and I started the 1, got to 2 and then she brought the cup back herself, at which I praised her all over the place.
It does work, you just have to stay calm, set reasonable consequences, and then follow through with whatever you said. Each of those three components are equally important.
Let's see, what else...keep at the timeouts, he will learn to respect them and if no other thing, timeouts are good for the parents as well as the kids.
Oh, and if he still laughs at you for attempting to punish him, 'punish' one of his favorite toys. My friend's 2 year old was doing the same thing, laughing at her when she tried to do timeouts and my friend found that putting her doll in timeout worked WONDERS. Not hidden in a closet, but out in the open where she could see it and be reminded, but not get to it.
Good luck!