Hi - my gorgeous Lhasa (who we only lost last year age 19) had this repeated times, it seemed to weirdly enough always kick in about this time of year and we never worked out why. (We're next to fields and wondered if it was somehow related to crops of something at this time of year - don't know though). He'd stop eating and stop bouncing around, vet would test him - blood test - levels through the roof, he'd go on a drip and antibiotics for about two days and no food (staying in at vets), it always looked precarious, and each time after a couple of days he'd start to perk up, vet would be amazed and he'd come home back to his normal self.
I think he had about ten or more bouts over his life, the first few were close together (every couple of months) and then it dropped to one a year, then it seemed to stop when he was about 13. He was on Hills I/d dog food and trypilase (enzyme powder) for life. He had many tests and ultrasounds etc, to check him and were told several times that it was touch and go when he was having a bout but each time he pulled through - he was a stubborn little bugger!!
By the end we knew the signs so well, that we'd have him down the vets in a couple of hours of the first sign and they'd have him straight on a drip and medication before blood results even came back - each time results when they did come back, proved it was that again. By getting him on quicker after the first few goes, he used to get over it quicker - first time was in for about 5 days, as I said by the end was about 2 each time.
He ended up being one of the oldest dogs the vets had on their records and they all knew him - we lost him to cancer in the end, so I know it's worrying, but he's in the right place and it is recoverable from and as my gorgeous boy proved livable with for about 15 years in the end (first dose was when he was about 4)!
Typing all that and thinking about him, has just set me off sniffing again - we have other dogs who I love but he was "the one" - enormous personality, determination, stubborness, loyalty and strongest will in the world all in a small fluffy body - he walked and acted as though he were a Great Dane (and no-one would dream of telling him any different). He's the one I will never get over losing! I do hope your boy is ok, I'm sure he will be - small dogs are amazingly resilient and I'm sure their stubborn personality helps!