I would trust your gut if something is telling you it's not right then get him back to the vet ASAP and tell then you want more options and you want more done for him DON"T EVER RELY ON A VET I LEARNED THE HARD WAY THEY WORK FOR YOU He should defanly have fluids.

If he's not taking any fluids i would give him some pydalight or gatorade and get him to the vet ASAP
You might also want him to stay there overnight if there's someone there all night long so they can montier him and check him every couple of hours

I really would get him back to the vet though and if this vet makes it sound like it' s no big deal Bring him to another one right away.

I hope he gets better soon