My son had a huge knot from his dTap when he was 6 months old. It was there for about 6 weeks. It got smaller, but I could still feel it. As long as he doesn't have a fever, bruising, or is having issues moving that leg I wouldn't worry about it.
If you are concerned I would talk to your son's pediatrician. They can always let you know if it's expected or to bring him in to get checked out.
Thanks Graysons_mommy, I asked them about it when he went back for the second part of the flu shot and she said that eventually it will go away and said for me to keep massaging it in the meantime. She said that it was nothing serious.
Good to hear! I am always worrying about my boy...so I always call his pediatrician with questions. I must have seen them 6 times in the first two months :)