Does he seem uncomfortable?
Is he vomiting or retching?
I would wait it out. It is very very rare for a stomach to rupture, and in those cases it is because the stomach is diseased. A normal healthy stomach can expand to incredible dimensions with no ill effects. I would just watch and wait at this point. It is very unlikely that he will have any problems with this besides a very uncomfortable belly for a while (sort of like at thanksgiving after the main meal!) which should subside over the next hour or two.
I hope this helps.
Sorry to take so long to get back to you. Internet is spotty where I am. I would worry if he does not seem to be getting more comfortable in a few hours (6-12 hours), or if he starts to act ill (vomiting, gums are white, crying in pain, etc).
I will be online for the next hour or so, if you want to give me an update on his condition.