Sf2, it will be no surprise to you that more info is probably needed. Has your dad had previous treatment that could have affected his pelvic lymph nodes, such as RP or RT? Does he have heart failure? Are his legs also swollen? Does he take water tablets? Could there be a blockage? How do you know it is fluid retention and not another medical problem?
Depending on the answers, he could have lymphoedema or the heart failure could be becoming critical - if the swelling started at his feet and has risen to his belly, I would get him seen without delay as his organs could be under enormous pressure. In fact, regardless of the questions above, if he were my dad I would be insisting the GP sees him first thing in the morning. I don't think you can mess around if there is any vagueness about the cause.
When Stan's fluid retention turned his toes and then legs into sausages, and then his groin area into a big pudding, the doctors drew a biro line on his chest and told us that if the fluid retention reached that line we needed to phone 999. Probably not what you need to hear right now but better safe than sorry.