What's already been mentioned about Reye's syndrome is the main thing. Aspirin (acetylsalicylic acid) is indeed linked to the risk of this rare but serious complication, when a viral infection causes dysfunction in the liver and brain. That's why in many countries it's simply excluded from pediatric treatment protocols up to a certain age.
Paracetamol and Ibuprofen are different substances, and while they also reduce fever and pain, their mechanism of action is different, and no such link to Reye's syndrome has been found with them. That's why they're used more actively in pediatric practice, but with dosages adjusted for age and weight.
By the way, for your son's ordinary headaches, simpler methods often help - water, rest, fresh air, a cold compress. If headaches happen frequently, it's more of a reason to take the child to a doctor to figure out the cause rather than just numbing it with pills.