What can cause severe abdominal swelling in a dog?

Go to a different vet. Have them run some bloodwork and take x-rays. Go immediately. If the regular vets are closed, find an emergency clinic.

You don’t mention if there are any other symptoms. If there is no vomiting (or attempted vomiting), it’s unlikely to be gastric dilatory volvulous (bloat), inteseception, or obstruction. If there is no coughing, exercise intolerance or tachypnia, it’s unlikely to be heart failure. Ascites as the result of liver failure is also a concern. Almost every possible cause of a painful, distended abdomen is a serious medical concern.

could be an impaction somewhere along his intestinal tract. Take your puppy in ASAP. Do not wait!

And, yes with syz’s comments: go to another vet.

Yeah seriously go have someone take a better look at your dog. Last christmas my dog’s belly started to bloat and she stopped eating. She threw up bile and then one morning i went to take her out and she didn’t want to move. Turned out she had a tumor in her stomach that had caused some blood vessels to rupture and made her bleed internally (that was the bloating). There had been no signs of the tumor until it was too late. She died by the time we got her to the vet.
Don’t mean to scare you but this could be something very serious.

@Nick; I am sorry for and worried about your dog. If you want to do some quick research on the way to a more careful and concerned vet, you are talking about Congestive Heart Failure in Dogs and not Congested. That alone is really serious, but as the more knowledgable here have mentioned, there could be many other issues.