It is possible he may have hurt himself. The only decor he has in his cage right now is a piece of driftwood and a cactus. But the driftwood isn't glued down so maybe he got his toe stuck under it somehow?
He could have hurt his toe if he caught it underneath of one his tank decors. It does happen though fairly frequently.
Liquid calcium usually absorbs better than the powdered calcium especially if they are having some trouble with absorption.
Everything sounds good with his diet, etc. Gout can be caused by genetics too & have nothing to do with husbandry.
Black cherry juice/extract should be able to be found at most health food stores. It definitely wouldn't hurt him any, even if he doesn't have gout. It is excellent for kidney health & overall health.
He would probably love the flavor, too.
That sounds great!
You can try it either way. He may like the flavor enough to where you can just use a dropper or syringe to let him drink it. It would also taste fine being mixed with food, also.