My first thought is to cut a large rectangle. Wrap the back section over the tube and attach it to the rectangle under the tube, this would be where his bum would sit. Then trim the front section narrower so it could comfortably slip between his legs and fold it over the front to attach under the tube. I would probably attach the pieces with the large heavy duty snaps so that it would hold in the water and any bouncing he would do and yet be able to be removed. This is a great idea, it seems so much safer than the arm bands or noodles.