This is Dr. Christian. Welcome to JustAnswer.
It is fairly common for there to be swelling where a shot is given, especially in the short period of time since the shot was given. You can try placing a warm pack on the area for 10-15 minutes to help the swelling. A heating pad set on low works well but don't hold it on the skin for more than 10 minutes since it can cause skin burns. If the swelling is larger or not improved in the morning then have him checked by your vet. If you have more questions please reply back.