That's great that he's able to pass stuff still! As far as solid versus less-than-solid, lol!: my corn tends to drop one or the other and there's no rhyme or reason to it that I've found. Kudos to you for going the extra step to keep an eye on him, rather than just writing him off and leaving him be.

I don't know about the pain part of things...I'm not sure how'd you be able to tell in a snake, short of him writhing or otherwise being aggressive or some other type of display. If you've done all you can at this point and he's still eating and passing as he should, maybe the store would discount him or give him away for free to a knowledgeable owner to keep so he could live out his days in a comfortable environment. Good luck!