now im not a doctor, nor are most people that will see and reply to this, but the video suggests it is trying to regurge it. The lump does look significantly larger than what a small rat would cause in a snake that size. My assumption is he is trying to regurge, but the rat is too far down to be brought back up, and because hes unsuccessfully regurgitating, it is causing his stomach to swell, like if you get a piece of chicken stuck in your throat after swallowing before youre done chewing, it takes a while and a lot of effort to get down and once you do your throat feels funny. My advice would be to take him to the vet, and if you cannot afford to do that in an animals time of need how can you properly care for animals? If you cannot drop 100-300$ at any moment for your animals well being, you may be unfit to care for another being life. I would pursue the previous owners for help, and if all else fails massage it.