Thanks for the advice everyone.
I'm not overly worried about him in terms of his health. Obviously, colic is worrying, as is waiting for the blood test reults, but his wees and poos have been normal the whole time and he's been eating just the same as always and he hasn't been thrashing about, he's not trying to bite his stomach, he just turned to look at it.
I've lost a bit of confidence in his owners who take care of him as often as I do, so had as much chance to spot the signs and more experience than I But they've spoken to the vet and I'm sure they'll keep in touch with him for as long as they need to. As far as I know, the vet isn't too concerned, it's just the horse is feeling sorry for himself and I want to know how best to take care of him (since horses don't like chicken soup) and cheer him up a bit since I'm normally the one who brings him treats and takes him for sneaky walks to eat grass (all OK'd with the owner who agrees it's not why he's poorly!)
Please excuse my spelling/grammar mistakes, I'm British.