Hi Pam;
just my 2 cents,
I would get a very high grade kibble, ones that are touted on here from other pug people are, Orijen, Timber Wolf etc.
He could be allergic to many foods or just one and it might be hard to determine what.
But perhaps a grain free would be the way to go first.
Pugs have allergies to many things and it may take you time to find out what Sully is allergic to.
My Bubba is allergic to anything with beef in it.
I have the guys on grain free and RAW and they are doing great.
A slow or gradual change over to a new food is more for his system to get used to it when a pug is fine with what they are currently eating; but if you suspect it is Sully's current food making him this way, myself, I would not hesitate to change Sully right away to a new food, as long as he likes it and takes it willingly. Some Pugs can be very picky eaters.
Try a search on "grain free" on this site; it will pull up all the threads pertaining to that type of food.
Hope this helps. Poor Sully, sad when they get that way

Do you suspect the food or could there be something else going on??