Thanks for all your messages.
He pooed on me in the pet shop when I held him but not since so it's not a reaction to me handling him. Last night he curled himself round my neck and seemed quite happy (possibly because he was trying to constrict me, but I like to think it's because it was warm). I've been keeping a close eye on him and making sure he hasn't been lethargic. He's a very active snake and he hasn't seemed to change over the weekend.
He's also stopped the pooing, nothing for two or three days, but I've made a vets appointment today to make sure.
I'll post again when the vet gives his verdict.