E.
I don't want to sound negative or skeptical, but man, you are lucky to be where you are with your son! I've had 4 babies, my youngest will be 3 years old in July and he still isn't even close to being potty trained! And all four of my kids were not potty trained at 23 months. The earliest I was able to potty train and be successful was my daughter and she was fully potty trained (no accidents at all) at 30 months. I understand the feeling of not wanting 2 in diapers. But it's not so bad. Also you have to know that when your new baby comes, this will be a big shock to your older son and even if he is potty trained by then, there's a very good chance he will revert back to soiling his underpants due to all the changes. Trust me, my 4 year old started soiling his pants again when the new baby came. If I were you, I would not push him and possibly put it off entirely until after your new baby comes and you get back into your regular routines.
Sorry to sound so negative, but my oldest is only 9 so I've been through this a lot in a short period of time! If you ever want someone to talk to just send me a message! Hope that helps!