What Causes Lip And Cheek Swelling?

I noticed a several months ago when my son (he is 32) was visiting that his lip was swollen I never said anything at that time but when he came back a few months later it still looked swollen and his cheek on the same side was thicker then the other - he had a beard the first time so I nevered really notice the cheek so when I mentioned it to him he was surprised also because he had no pain at all needless to say it still looks swollen (more like the tissue is thicker). He is in very good health.