We've just got back, and he's ran straight to his litter tray.
The vet said his tummy sounded fine. She had a look in his mouth, and said that she could see some sharp bits, but it didn't look like it was enough to make his tongue sore yet, although she couldn't see right to the back. He has had a painkiller and some antibiotics just in case, and I'm to take him back if he's no better. I do reckon it's his teeth, if it happens again soon, he'll be booked in to have them done straight away.
Update: he's scoffing hay, buy hasn't touched his SS yet.