Redness around an injection site is common after any vaccine. It comes from the local inflammation. In people who have darker skin pigmentation, inflammation disturbs the normal skin color turning the inflamed area lighter or darker than the normal skin color. Typically, normal pigment returns within few weeks, but it may take up to 6-12 months.
If you worry about how the area looks like, it gets larger, or bothers your son in any way, see your regular health care provider for an evaluation.