My Son, 28 Yr, Has A Large Hand-sized Soft Fatty

My son, 28 yr, has a large hand-sized soft fatty swelling on the backside of his top shoulder joint. It is quite large and soft. It doesn't hurt at all. It just looks like a large muscle growth under his skin. It is not discolored in any way. He hardly notices it. It is soft and I wonder if it is just fluid build up?