I had a similar scenario with one of my girls a few months back, and was particularly worried about a foreign object as she is a chewer, and it wouldnt have surprised me if she had half a sock in her gut. However, she was put on antibiotics and after 24 hours was more or less back to normal, so try not to worry too much at the moment, I know if you are anything like me you will be watching him like a hawk!
Did the vet mention scanning him or using an endoscope if he doesnt show any signs of improvement? If he is still being sick tonight, I would be inclined to go back and see a different vet and ask them how long he needs to be on antibiotics before they do any further investigation.