I think the smaller tennis-type golf-ball size ball you have would actually work well for Cloud on a simple flat surface such as the floor.
You can also look around for a soft "nerf" type ball or a super soft rubber type ball for him.
The main thing is if it rolls on top of him you don't want him to be hurt.
I have lots of super soft squishy balls on top of my birds cages because they love flinging them off onto the floor.
A couple of the budgies frequently try to balance on top of them as well. The picture makes them look hard and spiky but they are actually very soft and pliable.