Sorry to hear Storm has UTI. I hope the medication will help get rid of it. Instead of Hills C/ D which is supposed to prevent crystals forming, I'd give him S/D which should dissolve the crystals. Also, he'll probably like it better as there are more fat in.

It's very likely stress related as you think. Hopefully it will help calm him down when Cirrus finds a new home.

Good luck and scritchies to Storm.