First, congrats on the new baby. I hope you are adjusting well...i know how hard it can be.
I'm so sorry to hear about the shift in your son's sleeping. I have had the same problem on and off with my daughter who was an excellent sleeper until around 2 years old as well (she's now just over 2 1/2). I blamed ours on a move to a new home (she had a brother born when she was one) and we have been struggling for the last six months.
There were a couple of ways we dealt with it, all of which have helped a little bit but then stopped working (mostly because she has figured out how to get out of her bed - if she couldn't get out of her bed, I think we would have resolved a long time ago). First, she started sleeping with a few stuffed animals (not sure if your son does that already). Second, we started doing a slightly longer bedtime routine and even if the baby starts crying (I have a four month old), I keep to the routine and said things like "the baby has to wait because this is your time". We also made a list with pictures of our bedtime routine so that we could go through it together and it was clear what we were going to do and when it was going to be over.
On recommendations we tried music (didn't work - just got her geared up because she likes to dance), eliminating the nap (solved the daytime problem but made night-time harder because she was so exhausted), and changing her bed (you're now a big girl so you get a big-girl bed). They all helped for a little bit but she just wants to hang out with us instead of sleep.
The last couple of days we have tried the "your stuffed animals will keep you company while you try to go to sleep and I'll be right outside the door". Then I peek into the room every 15 minutes and if she is still awake I say "are you ok honey? Good night" and then close the door again. Just much easier than the struggle for that long. And I expect that eventually she will just stay there and know that we are outside.
Not sure that is helpful given that the circumstances are quite different. But thought I would share in case they spark something for you.
i hope this resolves itself quickly.