Hi welcome to HCM.
Your son must be having post traumatic edema spreading from forehead to the upper lid area. Although the pupils are reacting normally, at his age, he won't be able to express any discomfort in his eye.
Give him cold
compresses and a dose of anti inflammatory syrup for a day, look for any redness
or progress of the swelling, consult with your ophthalmologist to rule out traumatic inflammation of inner layers of the eye.
Hope it is just an upper lid edema. Take care.