I'm glad your doctor called you back. I had thought of suggesting you try giving him the pedialite popcycles, but didn't want you to think I was crazy since your son is so young. If you do that, please make sure a hunk of it doesn't come off for him to choke on.
It's not uncommon for little ones to have the loss of urine when they are not taking in any fluids, but I TOTALLY agree that if he doesn't have a wet diaper it is a good idea to seek medical help as babies will become dehydrated pretty quickly, and it's more dangerous for them because they are so tiny.
I will keep you and your son in my thoughts. Take care! I know it's scary when your little one is sick- especially when they can't actually tell you what feels sick.