B.C.
How old is your son?
His head is a big area - where on his head?
Face (forehead, cheeks, nose, chin,etc)?, top/scalp, near ears, back, near neck?
Is it a swelling related to a bump of physical trauma (he bumped his head on something?) or a swelling due to swollen glands responding to an infection (mumps, tooth infection, etc)?
Are the bumps big or small?
Are they pox (blister like) - chicken pox, hands,foot,mouth, measles, etc?
Any bug bites?
Does he have a rash, an abnormal temperature, is he acting sluggish, not feeling well?
We're not doctors, we haven't got a clue what's going on and since we don't have any idea what it is - of course we can't recommend any treatment.
Please take your son to a doctor, let her/him figure it out, and then listen to and follow her/his instructions.
If there's a high fever and/or he's unconscious - take him to an emergency room.