I think you're right, he's got a lot of things going on right now. Sounds like he needs a bit of extra comfort/reassurance. My ds is actually going through another one of these phases atm too. He's 21 months. I used to just pop him in bed, so goodnight, walk out & he'd go to sleep. around 8 months this changed. Since then he's had random periods where I need to be in his room for him to go to sleep. I started holding his hand (otherwise he'd just stand up & cry), then sat next to his cot on a stool, then further away etc. Sound like you don't need to do these steps though if you're already able to sit outside his door. Don't really have any words of wisdom, other than that he will probably go back to sleeping by himself again soon. Probably doesn't feel like it to you atm though.

My ds also started to sleep longer through the day at 16 months (1x 2hr nap), previously he only would have 2x 40 min naps during day. Don't know if this is relevant to your ds, but thought I'd mention it.