Is it only swollen around the site of the injection or is it a large part of his arm? A small amount of swelling around the site of the injection is normal...it will probably feel hard. Is this what it is?
My 5 year olds tricep swelled a lot and got hard and red, but went down the next day. I would say if it does not go away within the next morning TAKE HIM IN
Take the child to the doctor, and have him looked at right away. Especially if this is the first one he has ever had. I know my husband got his first one a few years back, and got sick from it. With someone your sons age, this could be far more serious. Hope this helps, good luck to you.