Welcome to health care magic. Thank you for posting your query.
I can understand your concern for your son. You are pretty anxious about him.
Let me assure you that there is not much to worry.
This lump on the forehead is perhaps a
bruise only.
Apply
cold compress (ice packs) over that area twice a day for 4 days.
It would have been great if you had mentioned his age or his body weight.
Still I can help you out.
If he complains of pain or cries out on touching , give him syrup
paracetamol (125mg/5 ml) 10 mg/Kg/dose twice daily for 4 days.
e.g if his weight is 20 Kg ,he requires 200 mg = 8ml = One and a half teaspoon like that. It is easy.
You can also take him to a paediatrician . He /she can examine him and correlate clinically.
Post any other question. We will be happy to help.
Have a nice day. Regards.