My son have a scalp hematoma since saturday and the bump is not getting smaller yet, whe do you decide if it needs to be drained?

I've never drained : One in 3 deacdes. They may take a few weeks to settle down & sometimes they can form a hard bony knot at the base of the collection.The body will eventually reabsorb the material & all will be fine. It is not worth the chance if infection or recurrent accumulation to drain these.

See a Pediatrician : Your son's condition needs further history and detailed exam before a pediatrician may decide how to proceed. Please kindly see his pediatrician.