Looks like he did wonderful! In Loki's puppy class we did the same exercise, with the peanut, regular exercise balls, and barrels placed on their side and with something underneath while upright to make it wobbly. One of the pics I posted of Loki months ago is of him on the peanut. A regular exercise ball is more challenging as you first need to make sure you have a good hold on it and then gradually loosen your hold so it wobbles however much the dog can handle at the time. And of course it is far more unstable than the peanut.
I haven't ordered the peanut yet, but now that you are talking about the regular exercise ball, I might go for that instead, because I can use it to stretch out my lower back.