How about free-feeding everyone the Hills?

Did your vet recommend it? If so, s/he must have known it was okay for Butter?
Ask your vet, hon.

I was wondering about your beautiful boy, and am glad he seems to be ok.

One thing - if Butter had crystals(which I think her did, right?), don't let him have regular food "once in a while", because that will turn into more often...and you will run into this again.

If Hills is too expensive to feed everyone, ask your vet about a suitable other food. With good quality food, the cats will eat less of it, so it really won't cost you much more in the long run.

And it will save you the expense and grief you have just been through.

Again, ask your vet for a recommendation...and go with that.

HUGS