Well it turns out I was right. And I've rarely been so happy to be right. Nothing anything scary going on, it really was just a loose baby tooth he'd been picking on. Yesterday morning (or night, sometime, really) I found something small and hard beside me in bed... I was soo sure it was a tooth - and that morning I looked around for it and found it. Yay!
Here it is, next to a AA battery..
It really is quite small, but it's still impressive how much of it is hidden...
At least now I don't have to worry about something being seriously wrong.