Hi K.!
It sounds like he's normal to go through more sleeping changes. I've noticed with my kids, that this is the age they "changed" routines......again :o)
I think you're lucky to have him actually fall back asleep still on his own, even though it's taking more time and patience on your part. If it were my kids, I would stick with what you're doing until he's "finished" adjusting through "whatever it is" he's going through.
At this age, I remember, my boys were "more aware" of their rooms. They noticed "shadows" and things which started to scare them at night. So, I actually would lay in there a little before they came into bed to "see" what they saw in their dark rooms. It was amazing how the different shadows/silhoette's showed up after the lights went out. I had to change things around, put them away different, always make sure the closet door was closed, etc..before they came into their room. Almost to the point where I was "cleaning" prior to bedtime. One of my boys had Night Terror's at this age, so it didn't really matter, 'cause I was probably getting up twice anyway. But it DID help with laying down and going to sleep...a little more anyway, mine weren't as good as yours about falling asleep on their own :o)
Keep doing what you're doing....it's sounds like he's doing truly wonderful, and KUDO'S to you for making him feel comfortable falling asleep in his own bed for so long! I would have LOVED that feeling of "accomplishment" with this area of baby-raising.
I think you're right to "hold off" on a Big Boy Bed until he's back to being "a secure sleeper" again. And if he doesn't adjust, well.......Mamsource will be here for you then, too!
Have a nice weekend!
N.