If the fluid was clear it was lymph. This is the only scenario I can think of. There's no way that anything other than blood is flowing through his vascular system. he's not cutting blood vessels, has to be absorbed lymph into his dermal region.
He could be having a problem reabsorbing lymph or one of the valves in his lymphatic system is not working properly. Therefore lymph is not draining from his arm, it is storing in his arm tissue and swelling. If it has been identified as lymph, he should be at least getting lymphatic message and wearing some sort of compression garment to ease the symptoms.
There's quite a few reasons why this might happen, a few I can think of are:
Castleman's disease
lymphedema tarda
Are they checking for PAD (peripheral artery disease)? Looks like they've checked his lymph nodes. And infection should show up systemically as well - blood work, fever. Hope he finds the answer soon.