It sounds like he's in good hands! It's unlikely that he was poisoned unless you think he got into something in the house - you can always call Pet Poison Helpline, a 24/7 animal poison control at 1-800-213-6680 if you want to start a case. That said, it sounds like he has pancreatitis based on the very elevated amylase level (pending his kidney values are normal). Did he get into any fattening table food recently (like bacon, chicken skin, etc.?). This is often the cause for pancreatiits in dogs. That said, it's often seen on ultrasound, and I'm not sure if this was detected when your vet ultrasounded him.
Nevertheless, the IV fluids, antibiotics, anti-vomiting and anti-diarrheal medication that your vet is treating him with all sound appropriate and hopefully will get him on the mend soon!
Best of luck to him!
Keep us posted!