Just back from the vets. She suggested he could have a blockage (he's a pup - he chews everything! so it's definitely a possibility) but she couldn't feel a blockage and he wasn't at all bothered by her poking and prodding him so she thinks he's just picked up a bug of some sort but we've got to keep an eye on him and take him back in the morning if he hasn't improved. He seems to have perked up a bit, and she said he's not really dehydrated so he should be ok. Poor little pup.