My guess was the same as Mandy's - he may have gorged himself so much that he's just completely pogged. That kind of Christmas evening feeling. There is the possibility that he's eaten something stupid like some rubbish (plastic bag or something), but more likely he's just stuffed himself silly. Just keep an eye on him and see how he is tomorrow.
Presumably the vet checked his temperature? If not, do this yourself and see if it's normal. Check his faeces if you can. That might give you a clue if he's eaten something he shouldn't. Unless he's seriously underweight, don't be too concerned about him missing meals.
Good luck and keep posting on his progress. Be interesting to see how things go.