A question for natural remedy experts about my dog

We adopted a healthy puppy with a mix of boxer, lab, and pit. He was the completely healthy, strong, and had a great coat. His issues started after a vaccine he got real lethargic and stayed in a dark room for a few days. He eventually got better but 2 months later he swelled up mainly his back legs. They put him on and off prednisone as the symptoms came and went just to help the swelling. Fast forward 2 years of this he now has swelling over his whole body they recently pulled 2 litters of liquid from around his lungs to help him breath. He eats very little and hardly walks. Keep in mind he's only 5 years old. Seen 3 doctors non can give a reason. The best answer I got was heart disease but that don't make sense to me since his heart was checked when they all started and only recently as his health went down has it effected his heart. They all mentioned quality of life for the dog and suggest ending it. I agree but I wanted to check here is there anything I can try before I put the little guy out of his misery? We are going to give it another week (assuming he's not in too much pain) and see if he improves. I am trying cbd oil I just started it yesterday about 15 ml for 2 days now. The vet put him on anti-inflammatories and pain killers. Any advice will be appreciated.