I was having a problem with my ankles swelling due to my lymph system going sluggish. If caught early, it can easily be remedied with reflexology. If not, it usually takes some medical intervention (because the swelling can get SO bad that you cannot move the legs...at least that is what happened to me). Maybe take him to see a reflexologist/chiropractor just for some more insight/opinions.
In my case, I was put on all kinds of steroids, etc the first time this happened to me because the docs couldn't tell me what it was (I, too, had every test imaginable run and everything came back negative). It had gotten so bad that both of my legs were swollen to the point of my skin looking like glass and not being able to bend my knees. I finally went to a chiropractor who specialized in reflexology/accupuncture/accupressure and he started "lymph" brushing (stimulating the lymph system) and gave me all kinds of accupressure/reflexology methods to get the lymph system going again.
The first time took quite a while to alleviate, but it worked. The second time it happened, I caught it right away and did everything from before and while I had some swelling, it wasn't anywhere near as bad as before.
I would suggest in the meantime to elevate his feet as much as possible...at least for a couple hours a day. Vigorous chest and back massages. That's the start of it. If you need some referrals for some good chiropractors who may be able to help (at least give you some opinions/options) please let me know.
Hope your son is feeling better soon!! Please keep me updated!!