Maybe next time you take your doggies for a vet check (heck, invent one! )
bring back note from the vet about the "diet" they have been put on. Hopefully you will have several pieces of paper from the vet, and maybe your FIL will have a look through. "Oh, yeah, he gave us stuff about the dogs, I haven't read through it yet.", and hand it to him or just set it down somewhere.

Or - "Don't ask me, as your vet." I assume they used to have one?

HUGS!