This was on his 5-month birthday, 2 days ago. You may note that the book is open to a page that says, "Baby's sitting. Wobble, wobble." At the time I snapped it, he was very wobbly, but 2 days later he can already sit without immediately falling over! (When put in that position.) Also, when he does fall, he can either catch himself on his own arms or flip into a tummy position before he finishes falling all the way. Strong kid!
I swear he will be crawling any moment. He can already roll all around, move in circles, and scoot backwards. But he also gets up on all 4's and rocks violently--almost hops like a bunny when he's doing it! In comparison, I think my second son crawled at 7.5 months and my first crawled at 8.5 months. Oh yeah, he's also really happy again and sleeping through the night again. I guess he was frustrated about not being mobile enough, but now he is too mobile!