Assertion as anger management
There would be many ways you could respond. I might take the child aside to gain some private space, crouch to go face-to-face and tell him that his behavior is unacceptable. You are angry because his belching shows disrespect for others in the store and makes you both look like stupid people. You will not allow him to continue.
Now there is no way to reasonably stop his behavior. He has to make the choice to stop. Once you draw the line, some children will test your perseverance by continuing. If he does, you have no choice but to immediately leave the store and return home. Do not give him a second chance. If he has done this before and you have had this confrontation at that time, you should leave immediately with the very first belch.
The main point here is to use your anger to oppose an unreasonable action. This course is on anger management, not child discipline. If you want to take an on-line course on discipline, visit my Responsive Discipline page.