Poor Tubby, I guess he isn't just "lovesick" after all if he had to go see the vet!
Maybe he just had too much excitement last weekend with all the other cats visiting.

I am sure that Phred will have some good advice for you, I remember him mentioning giving tuna juice drained from a can of tuna to help hydrate Boots.
Maybe Tubby will like it too.
Good luck in making him feel better, get lots of rest sweet Tubby and feel better soon so you and Samantha can make plans to travel someplace warm and sunny for Valentine's Day.