Don't push that physical envelope too far! If you can, get him a Desk Cycle instead of going for a walk. Isolating muscle groups instead of using all of them for walking is better when someone's MG isn't stable.
He still needs to be assessed by a sleep pulmonologist for his breathing issues. He needs to know if he has sleep apnea!! If that goes untreated, it can cause brain and heart damage.
I hope he continues to do well.
Annie