Little Man is sitting! He finally figured it out. I still put a pillow behind him, just in case he topples backwards, but he has sat up unassisted for as long as 30 minutes now.
THANK GOODNESS. I think he will be happier now that he can sit up, look around, reach for toys, etc. And he and Sweet Pea are really starting to interact, which is so cute to watch. I sit them down, facing each other, and they start laughing and reaching for each other's faces. I'm going to try to get it on video one of these times.
I attribute Little Man's new-found sitting prowess to this:
It's the Leap Frog Learn n Groove music table. Even CC loves it! Every knob and button and whirligig plays some kind of music or makes a sound. It says the ABCs, counts, giggles, sings, etc. Right now it's just sitting on the floor, so they can reach it easily. But once Sweet Pea and Little Man are standing up, I can attach legs so they get even more use out of it.
I really like that it's big enough for two (or three!) babies to play with at the same time. I sit Sweet Pea and Little Man at opposite corners to keep the table in place, and they have a blast.