Sitting or falling?

The littlest one loves tummy time and can roll from tummy to back and back to tummy, but he isn't much interested in sitting. A little while each day, I sit him between my legs and let him go at his toys in a different way. He likes it, but has no balance. (No worries, he is only 5 months old.) I tried to have him sit on his own--just to catch a picture. No luck. This is what I came up with. The funny thing is that when I started pressing the button he was sitting.

I really don't want to hurry his growing up--he is already too big--but it is fun to see what he can do.