Quick update:

I showed DH the advice here and he said he really thought about how he was riding when he was out this morning. He said that he noticed coming down a (different) hill that going down the hill only he is pointing his toes down. He said it felt like perhaps he was pushing his toes down to try to pedal fast enough to feel resistance and that was starting to cramp his calf. As soon as he remembered to pedal with his heels down the cramping went away.

Thank you so much for helping us!