If the owb has him they are both flatfooted.
That's it. There are no skill checks that the cleric themselves have to make. The owb doesn't require checks because it has perfect maneuverability.
He would have to command it verbally without using a custom made item.
Stop making things more difficult then they have to be...
If he actually ever wants to ride it have him talk to a wizard with the craft wondrous item feat and at least a 10 spellcraft. They can make an constant effect enlarge item to make the owb large size category that way.
It will take a while since he'll have to cast enlarge 100 times.