Welcome to Purrs, sorry its not in good circumstances for your cat.
If Storm is still being sick and having probs you need to speak to an emergency vet now.
Next Tuna is not good to be fed regularily to cats and secondly in brine is definately not good, as it is too salty. You need the type in springwater and others will advise but probably no more than once a week. However, how long have you been doing this because you may need to ween him off it as I am sure someone has posted in the past that cats become addicted to it.
I do not know whether this has any relation to your cats current problem and suspect not but if a cat is contunually sick and urinating a lot in this weather he will quickly become dehydrated.
Please speak to a vet asap.