Dylan has always liked to go upside down. And, he's enjoyed yoga off and on since he was a baby and I did "baby yoga" with him (from the book Itsy Bitsy Yoga).
He started doing downward dog when he was a toddler.
And recently, one of his ABA tutors taught him to call his upside down pose downward dog and also taught him how to do the seal pose (actually a cobra, but much more fun when you can say "arr arr arr" like a seal). He really liked doing these two poses, so for Christmas I ordered him the book "My Daddy is a Pretzel". He has learned all the poses and will ask to do them by name when we aren't reading the book. His favorite pose is the Plow Pose, and it seems to work as a calming distraction when he gets upset. It typically makes him giggle. Here is Dylan doing some of his yoga (and I should note these were hard to catch because he wanted to see each picture before I could even take them. When Denny got home, it made it easier.):