Sitting up & naps

C

My LO has learned to sit up on his own. This has made it increasingly difficult to get him to nap as he doesn’t want to stay on his back, but rather get up onto his bum. We went from having 1-2 hour long naps to 30 min naps until he wakes up. Help! How can I get him back to napping?! He’s also not super at sitting yet so he keeps falling over and whacking his head on the crib.