Most likely it is hypoglycemia, Maggie. Go to this website for more information and what to do about it.....then take your baby back to the vet and discuss this with him.
http://yorkieviews.com/healthfacts.htm#HYPOGLYCEMIA
I know how awfully worried you must be. I hope he is better soon and that it doesn't happen again.
We're glad you came to YorkieTalk....please keep coming back and DO let us know how your furbaby is doing.