My son is 13 and hit his head hard against a wall playing basketball. is this serious as he has no symptoms and only has a lump that hurts when touch.

Keep watch x24 hours: Fortunately the brain fits more solidly inside the skull of a 13 y/o comparing to a 73 etc... So often bumping head against hard surface causes a big bruise/bump which will resolve. Since he has no other symptom, he should be ok. But, i would keep an eye on him--if has increasing headache, nausea/vomiting, dizziness over next 24 hrs, contact doc asap. Otherwise, he should do well. Good luck.

Probably okay: If he had no loss of consciousness or confusion after the injury, he is probably okay. It doesn't sound like there was any associated brain injury if the only symptom is a lump that hurts.