I never fast my dogs if they are sick - they will do it themselves if they don't feel too good, but sometimes bringing their food back is just a reaction to eating too quickly.
If he is going to the toilet ok then its not usually a problem, and within 24 hours they are back to normal. If either sickness or the runs lasts for more than 24 hours then take them to a vet as they can dehydrate quickly. He may just have eaten something that didn't agree with him or it could be that the change of food upset him although that is more likely to affect his poo than make him sick. My GSD had a day last week when he brought his dinner back a couple of time and was a bit loose, he didn't eat his tea that night but by the next day he was back to normal.