The only way to tell is to lift the skin at the back of the neck. It should fall right back in place when you drop it. If it doesn't, he's dehydrated. But this is hard to see if you don't know what to look for.
For tonight, I suggest that you give him plain pedialyte with a syringe in the mouth and this will help if the problem IS dehydration. Then see your vet in the morning.
Let me know if you still have questions.