Hi Amy,
One thing that is hard right now with potty training is that there are pretty much two schools of thought: (1) Elimination Communication, where the parents are in tune with when their babies have to go, and babies are "diaper free" from early on, sometimes infancy; and (2) Waiting until the child "self trains" and is "potty independent" right off the bat, often at 3 or even 4.
We don't hear too much about anything in between these days.... But ask anyone who was raising kids in the 60's & 70's (or earlier)--there weren't musical toilets or Pull-ups, and there also weren't a lot of 3 and 4 year olds still in diapers, either.
Much of what route you take depends on what you mean by "potty training" and how much responsibility and work you want to take on. If you are willing to become in tune with his potty rhythms--basically put the responsibility on yourself to get him to the bathroom at the right time, do extra laundry, put in time and effort and have the patience, then yes, it can be done now. If your idea of "potty training" is him being able to say, "Mom, I have to poop", and agreeably going to the toilet to do so, and not having to deal with too many accidents, then no, you probably need to wait.
There is no "right" answer here--it depends on what you want.
I will admit, I am a fan of introducing the potty chair earlier rather than later. I think that it can be a part of the child's daily routine and something they get used to, even if they are only using the potty part-time. Also, some kids do not "self train", so you could be waiting until hell freezes over. My brother was in diapers off-and-on until he was 4 years and 10 months simply because he was allowed to be.
If you want it to happen now, get him out of disposable diapers during the day. Pull-ups are a gimmick. They are expensive size 6 diapers without the tabs. They keep kids dry, and often totally unaware of when they pee. And what incentive do kids have to use the toilet if they can just go in their pants and still be comfortable. Put him in training underwear (thick padded underwear) or regular underwear with plastic training pants and don't look back. He then has a choice--to use the potty and be dry and comfortable, or to go in his pants and be wet and have to change. It will click with him, but if he has been in disposables up to this point it may take a few "wet" days.
My son is 17-months-old and has been using the potty part-time for 6 months, so YES, it can be done. One thing that has helped is we use cloth diapers. My son does not like sitting in a wet or poopy diaper--who would. He caught on real quick that if he went in the potty, he wouldn't have to be uncomfortable. He goes poop in the potty almost all the time--we can go weeks and weeks without a poopy diaper. It is SOOO nice not to deal with poopy diapers. And we are down to 2-3 wet diapers/day--the rest of the time we make it to the potty. He does the sign for "toilet", but most of the time I know when to take him. I will admit that right now I am the one "trained", however, I am also "trained" to feed him at certain times and "trained" to bring him to bed at certain times, so that argument doesn't stick with me. All I know is, I don't have to deal with shelling out for expensive disposables or laundering piles of cloth diapers!
Whatever you decide, make sure you are ready and up for it, and that there is no stress going on in the home. I would say either start now, or wait until a few months after the baby is born. Good luck to you.