First, don't panic.
I know it's hard but it sounds like he might have picked something up at the spa - like a flu. And what happens when we get the flu? We don't feel like eating or doing much of anything other than being miserable.
I don't think you need to worry about dehydration immediately if he's not vomiting or does not have diarrhea however it's good to keep an eye on things and make sure he's not getting too dry - a good place to look is at the gums!
I would expect to see him perk up a little bit in the next 24 hours and start eating and drinking normally. If he does not, I would call the vet.
My bulldog Bentley had very similar symptoms once and it was a bug that eventually ended on its own after some rest and TLC.
I hope Bugszy feels better soon, keep up updated.