My DS is nine and we laugh about his ability to fall over fresh air!
Like your DS if there is anything at all on the ground you can guarantee he will trip over it, his own feet included.
I think it is because he is so focused on whatever he is doing at the time or going to do that he forgets "left foot, right foot, breathe in breathe out, left foot, right foot ........"
20mths is still little so I wouldnt worry yet. How does his eye hand coordination seem?? DS is a shocker on his feet at times but can catch a ball from anywhere anytime with no sorries at all.