He may have pancreatitis. Yorkies are prone to it.
Pancreatitis (Inflammation) in Dogs
I would definitely take him to the vet first thing in the morning as I am sure at this point he is dehydrated at the very least.
If he gets lethargic before your vet opens, though, I would find an ER vet. If it is pancreatitis, it can be very serious.