Haha. My dear son has been doing that butt up thingy since dont know how old until now at times still doing! LOL. When he was 5mth+ coming 6mths, he learnt to flip.. So kinda dangerous flipping on bed as he can push and kick the pillows down.
So, I got him a bumper mat from minitoons and let him do that on the floor.. At least he won't fall off the bed. But one thing I find that the mat is kinda rough coz it has little bumps thruout the whole mat.. Like anti-slip mat material..
So far, he's still fine with it... Haha.