Well, birds don't burp as they don't have the same type of digestive system. What I think you were seeing was him grinding his food. Take a look at this picture and this paragraph- it might help. If it's not an actual lump that you are seeing all the time and his behavior is ok I wouldn't worry about it.
"Birds has special digestive tracts because for the most part they do not chew their food. All breakdown must take place within the system itself. First the food travels down through the esophagus and into the crop. The crop is actually just an expanded section of the esophagus. It is used as a holding tank for food before it can enter the proventriculus. The proventriculus produces acid and adds enzymes which aid in breaking down the food, which is next passed to the gizzard. The gizzard helps grind food tougher food like seed............"
Courtesy of http://sprout.phpwebhosting.com/~feistyhome/anatomy.htm