What Causes Painful Boil On Testicle?

My husband has what starts out as a small boil on his left testicle. It eventually gets very large and hard until it eventually stretches so large it pops and drains. It is very painful, difficult to walk, sit, wear clothes,or sleep due to the pain until it pops. He gets these quite frequently (he has had two in the last 5 months). He lays with warm compresses to it until it pops but gets very uncomfortable with them. What causes him to have these so frequently?