Sorry to hear about your poorly cat and father in law!

As tothe cats diet i'm going to echo the advice about a water fountain, many cats love drinking from running taps etc and a fountain emulates that!

For the diet i would also suggest a high quality wet food, something with a high meat content of around 60% minimum (hi life, forthglade etc). You can buy cat toothbrushes etc to keep his teeth clean. The dry food keeping teeth clean is a myth.

hope he's better soon xxx