They sure are the most wiggly of animals! I had my Littlest Mac on the kitchen counter when he was 10 weeks old and while holding him - he squirmed out backwards between my arm and my body and yes he fell to the floor and started yelping like there's no tomorrow. Scared me 'crapless'! He (and I) finally settled down and I could see that he moved okay but I still took him to the vets for reassurance. My little guy was and is perfectly okay and I'm sure yours will be too - but it sure is frightening. You may want to take him to the vets just to get reassured as well; just so you won't have to say: "I should have taken him, I know I should have"... Hindsight is 20 / 20 they say.
Better safe than sorry later.