The vets have deemed it not to be laminitis.
He didn't show any reaction to the hoof testers and he actually trotted out sound. The vets thought that he might have a bruise that was just too deep in the foot to show anything when rapping on his hoof (or mystery lameness, they weren't quite sure). His soles are really flimsy so they figured it wouldn't be too difficult for that to happen. It seems I really do need to look into hoof boots (and plan how to set up the pasture to harden up those hooves when we move).
They did think the hay situation was a little suspect and that he should go easy on it for a while. I did get the go ahead to start riding again. So I'll see if he can handle a little walk tomorrow.
I am SO relieved right now. Drinks are on me!