Where exactly is the swelling?
It could be caused by a sprain if its at a joint, I personally would get it looked at just in case it needs treatment. To help with the swelling, get him to keep it elevated and use hot and cold compresses. Put on a cold compress using a bag of frozen veg wrapped in a tea towel. Leave it on for about 10 minutes then use a hot compress such as a hot water bottle wrapped in a towel or a towel heated up on a radiator. If it continues to sweel, get it checked out at your local A&E.