My horse tends to hollow out his back a bit while I'm riding, which results in a stiff neck and his head in the air. Usually he does this when we ride outside. Inside he will put his head down and I can really feel him trying to round out and push with his hind end.
How can I get him more focused and more rounded out?
I will add some pictures to show his "hollowness".
(Excuse my awful position. )