OK, I need to clarify that I'm telling you this because you want him out of your room and sleeping in his throughout the night. There may be those who tell you he needs you, it's OK for him to co-sleep, he is experiencing anxiety about the new baby, etc., but that's not what you indicated you desire. So, you need to let him know that tantrums (the crying, screaming, saying he needs to go potty 10 times an hour, etc.) will not get him his way at all. You were on the right track but you caved. You need to be consistent, he needs to know that you're not going to give in no matter what.
It's going to take awhile and I'm sorry you have to do it now that you have a tiny one, as it would have been easier before, but if you really want him to sleep on his own you'll just have to do it.
You've already laid with him, sat in the chair, then moved out of the room to do what you need to do. So now you have to sit him down during the day and say something like, "From now on you're going to go to sleep in your room, and stay there. We're not going to lay down with you or sit in the chair. (You need to take away his bargaining tools.) We don't want to hear crying and screaming, and you need to stay in bed. You'll go potty after your bath, putting on your pj's, and brushing your teeth, and that's it, you're not getting up again and again to go potty (another bargaining tool.) You don't have to go to sleep, but you have to stay in your bed and be quiet. Everyone sleeps in their own bed and room at night." He understands, you've said that yourself.
Then you or your husband park yourself outside his closed door after your "good nights," and when he opens the door you calmly take him back to his bed, don't turn on the lights or engage in conversation, just kiss him and place him in bed, turn around, ignore the crying and go have a seat outside the door.
After the first time you don't say a word to him as you take him by the hand (or carry him if need be) to his bed, turn around, leave and close the door. Be prepared to do this all night if necessary, which is why I'd start it a night your husband doesn't need to go to work in the morning so he can help. You need to let your son know that tantrums will not get him his way at all. Each night will get easier, though at the time it may not seem like it. And when he gets to the point where he sleeps most of the night and then wakes and comes to your room you have to take him back or it all starts over again in his mind.
Make sure he has a lovey of some sort to hold through the night, and tell him he can talk to it. Don't reward him for staying in his room all night, you expect him to.
Here's an article that goes into depth of what I've said. The author dealt with the same problem and co-wrote the book "The Sleepeasy Solution" which is excellent, and she refers to what you are needing to do as "boot camp." She mentions that potty-training would be a reason not to sleep-train at the same time, but as he's been "on and off" I would concentrate on this and then deal with the potty training later as the discipline would more than likely help him there.
http://www.parenting.com/article/how-to-get-your-kid-to-s...
Take naps if you can, and take back your nights ; )