Hello A.,
Congrats on your pregnancy - how exciting! My daughter was 2.5 when my son was born. I first off just wanted to say that two in diapers - not a big deal at all. I remember for a short while fearing it, but then realized really how easy my life was because of it - no rusing to the potty while at the park with newborn in tow, etc.. Personally it can be easy or difficult either way, just depends on who you are talking to. My son showed signs of rediness at 18mos, I did nothing to encourage it as he was really to small to get himself up and down from the toilet and remove his pants/diaper quickly enough but he managed anyway here and there. Then he stopped playing around. He was officially potty trained at 3, but not before me making several attempts at training him - little did I know then, he just wasn't ready. This is why I believe we never "got it". Shortly before turning 3, I came across The No Cry Potty Training Solution by Elizabeth Pantley - I've recommended this book so many times here at Mamasource. I feel so strongly about it, it is a short and very informative read. You'll find out if your child is ready, if you are ready and if so then you'll get some easy steps to make it a success. When I read it what I learned was my son was more than ready, it was me. Once I was on board, he was "trained" in a few days and has never looked back. When kids aren't ready, potty training can take up to 6mos or they may catch on quickly but then regress. With my first child, my daughter, she actually started potty training around 2, then I had my son and she regressed. Then we moved and well, you see where I am going. She wasn't potty trained until she was almost 4. If you still feel your son is ready and it's not just you not wanting two in diapers, then go for it, just stay consistent. If he is going to be in underwear than you must take him potty on a regular basis - set up a bathroom routine, he will get used to going to the bathroom at these set times, every hour or so at the beginning. As for cotton underwear that is semi waterproof, haven't heard of it. There are re-usable "underwear" I got mine at One Step Ahead and my kids sometimes wear them at night. They will not hold a lot of pee though. My kids are 6 and 3.5 and wear pull ups at night. Night time and day time potty training are different. Most kids do not stay dry at night until the ages of 5-7. Yes, some do stay dry at night sooner but most do not, so don't confuse the two. Just work on day time for now.
Lastly, I understand your frustration, I felt it as well, but honestly, once I read the book that I recommended, I "got it" - you really must read it. You don't want to be frustrated or stressed when trying to potty train a child, especially one that isn't really in to it. They pick up on everything and really he isn't doing anything wrong. It's totally normal for him to still be in diapers and I swear, I'll say it one more time, aside from the expense and I know it's and expense, two in diapers isn't that bad.
Congrats again on your soon to be born baby girl!
M.