They would probably prefer that he suck it, especially if they are looking for a h type fistula, though they may use a gastric tube. How long it takes depends on what they are looking for, how fast the barium moves through the digestive system, how cooperative he is, an the quality of the pictures they get. It is not uncomfortable for them, but they do have to be reasonably still. If they need to rotate the table (to move the barium around), he will be strapped to the table very securely. Ask the radiology centre if you are allowed in as some don't let parents in procedure rooms where radiation is used.