The one I've heard most is get down on your hands & knees & scrub the floor.
If I were you I'd give it a few weeks though. You don't want to do all that work to get bub head down to have him turn again a week later.
Maybe give your little ones piggy back rides. Not a good idea, but I spent a bit of time everynight on my hands & knees to get DS into position & DD2 was always sitting on my back. I just never moved.
The exercise ball is a good idea too.
I also filled my bath right up & layed belly down in the water over the end of the bath. That way I kinda floated there. I have a good sized bath though!
GL. Hope he turns soon.