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He could simply be constipated or he could have an intestinal blockage that is causing the problem. There is also a condition called gastric bloat that is a concern in these dogs.
You can give him 2 tbsp of canned pumpkin (not pie filling) with a meal and see if helps to move his bowels. If not, you will want to involve the vet.
Let me know if you still have questions.
You should also be aware that this swelling could be caused by fluid in the chest wall. This is most commonly caused by a heart problem. If you don't see some relief from this soon, you will want to see the vet.