Hi Melissa, 20 months seems to be a magic number!
You are right that there is lots going on developmentally!!!! Odessa kept giving it to me in waves for nearly 3months. Good week, bad week, good week, 2 bad weeks
What is his total sleep and when does he wake up for the day? I think stretching him out at both nap and bt would be worth trying. I tried to tweak odessa during this period. Sometimes it worked and paid off, so I'd repeat what I did the following day and she'd prove me wrong. So it was like a stab in the dark most of the time. And typically for us during a regression, it didn't matter how much i tweaked the schedule, she would still take the same amount of time to fall asleep. So it was very confusing me.
Having said that I stuck as much to my "schedule" as I could and slept as much as I could (I know this would be hard for you with a newborn too!!! (((((Hugs))))) and took the EWs on the chin for as long as I could.
Then for no reason, she started to sleep more predictably again, although the schedule was all wrong and we still got EWs. That's when I started to shift her. Also I noticed for odessa that after each growth spurt a schedule change was required... Why didn't I work this out earlier?!

I found this regression to be so much longer, so I began to start worry about the schedule, but in hindsight she was just growing and there wasn't much I could do apart from being consistent.
I think if he's taking 30mins to fall asleep, set nap for 1315ish and push out bt by 15 mins also. Is he still giving some cues? If he looks good, you could go to 1330. If he still takes 30mins to settle, then it maybe something else.
I'm no expert, I've just done all this in the last week and still shifting. But, I know for a fact that my daughter is not growth spurting at the moment and for us that is a big part of this process actually working.