Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Bo has a swelling/lump on his leg (side of elbow) that you believed was a hygroma and now has fluid being retained throughout his leg.
This could still be a hygroma. A hygroma is a false bursa or fluid filled sac with a thick capsule. These are common in large breed dogs that lie on hard surfaces and can get quite big. These are seen most in rapidly growing very large puppies or old, boney dogs that lay on hard surfaces a lot.
They form over boney joints that aren't covered by a lot of tissue such as elbows, the side of the hock, very rarely the hip if the dog is quite thin. They aren't painful.
If this started as a hygroma you can decrease the size or at least stop it from getting much bigger by having him lie only on padded surfaces.
They can be drained but if the pressure on the joint isn't relieved then they will come back.
If they get very large or ulcerate surgical drainage with a drain left is attempted but it is hard to get them to heal and they can get infected secondarily. That may be what is happening with him. The infection may be causing swelling in his entire leg because it causes inflammation, and can interfere with proper lymphatic drainage. So I would focus on padding. They form due to the bone rubbing against the soft tissues overlying the joint. The fluid accumulates due to irritation.
You may need to confine him in a crate with padding for this to resolve.
They do sell padded bandages that he can wear too to protect it, but many dogs don't like them, chew on them and then you need to place an e-collar. These aren't harmful at all unless they rupture (pretty rare) or get infected which may be the case with your fellow.
Both are more common in old dogs with fragile skin.
At this point though because things have changed for him it is probably best he see his veterinarian to aspirate some of the fluid that is collecting and culture it, and start him on antibiotics in the meantime.
Your veterinarian can examine him and aspirate the lump. If this isn't showing signs of an infection on the aspirate it is possible that this isn't a hygroma at his age however. This could be a true tumor. Tumors tend to be more solid feeling, and they are more likely to interfere with blood and lymphatic flow, thus more likely to cause whole leg swelling.
Aspirating the swelling could allow us to characterize this as a tumor and give us an idea of the type of tumor it is.
Possibilities include sarcomas, carcinomas, a hemangiopericytoma, or a mast cell tumor. Lipomas are also a possibility, but they tend not to cause a lot of inflammation and thus are less likely to cause fluid accumulation in the rest of the leg.
Best of luck with your fellow, please let me know if you have any further questions.