I am sorry to hear you are worried about Buddy. I am a veterinarian and will do my best to help.
The swelling you describe seeing from his front legs to his scrotum sounds like it is between his skin and body. It could be fluid that leaked out, after the abdominocentesis or it could be "dependent edema", fluid leaking out of his blood vessels and following gravity to the lowest area of his body.
Do you have any recent blood tests on him? If his blood albumin is low, then dependent edema can form, as well as the intra-abdominal fluid that was removed by abdominocentesis. Heart disease can also cause fluid to build up inside the abdomen as well as the dependent edema.
The round nodules in his chest could be tumors, or they could be some kind of infection. What you describe is what we see when cancer is spreading to the lungs, called pulmonary metastasis.
Is he anemic (low red blood cell count, pale gums)?
If you have any blood tests you can show me, that might be helpful. Rebecca