Edema (swelling of the lower extremities) can be caused by loss of kidney-function. Has he has a kidney-function test to determine if his renal function is impaired?
The cyst can & will cause a bit of pain and discomfort, but as there are no calcifications present, I would say that the risks of the cyst actually being present are minimal.
Ask his doctor where on the "Bosniak Chart/Table" his cyst falls. If you Google "Bosniak Chart" it'll give you a lot of information.
I've had NUMEROUS renal cysts that are Type-2 on the chart, and have had them removed via laporoscopic cyst decortication, and have recovered nicely. They tend to reoccur, and I live with them and the discomfort they produce.
The cysts, from reading what you wrote above, I would say aren't the concern. The leg-swelling, however, is a little cause for alarm. Don't panic, mind you... just talk to the doctor and see about getting a renal-function test done. Edema can cause complications, and I'd hate for you guys to be caught off guard.
Good luck and post back when you guys find out what's going on.
(I'm not a doctor, but I have a lot of experience with kidney-function, cysts, kidney-stones, etc)