My son at the same age was doing the same thing, I too was pregnant and now we are a year down the track, my advice is to remind you that you are the boss. At this stage of your pregnancy your son knows something is going on, you look different, people may be treating you different, things around the house might be changing and he most likely doesn't like it. By driving you mad he is getting all your extra attention, even if it is smacking and screaming, you are focused on him at the time. Also I find 2 year olds are all about "me, me, me". I spoilt my son at time of the birth of my 2nd son, he used me to the ground. He refused to go to sleep unless I lay with him, I was getting 2 hours a sleep a night when the baby came. His jealousy was not with his brother but me. I tried everything in the nicest ways, but he just got worse, we couldn't go anywhere. In the end I had to turn the house into bootcamp, what I said was final, if he misbehaved anywhere, I picked him up and left. Left half full shopping trolleys, left playgroup as soon as he misbehaved, no second chances. It took 2 weeks and loads of tears, but once he realised he was missing out, he just got better and we are best friends again. Good luck, your son may just turn back into an angel after the birth of your second child, I've seen that happen too, but don't let him be in control of you, your mum and he needs to repect that.
Jo, Sydney mama of 2 boys