my b thankseagle is two and a half years and has a problem with colitus,he has been ill for 3 days and not eaten anything,the vet said it might be his pancreas, im worried sick anyone got any advice,how much should a 13 inch beagle be eating

I think you really need to be advised by your vet here as if it is pancreatitis or colitis he will need a special diet, So it isn't so much quantity as opposed to type of food that is an issue. I think with pancreatitis it is a low fat diet. As you are living with the colitis I guess you already know about the diet requirements for that.

Has the vet taken blood to test for pancreatitis?