My son fell against the corner of a wall on his forehead, the spot is quite swolken still but soft. should i be concerned?

Depends on details: If a child is acting differently, even after a mild injury, he should be seen by his primary doctor. Sometimes an ER visit is needed, depending on the symptoms. Head bumps in which there is low-impact on the head, little crying, no changes in behavior and no headache after the first hour, etc..., usually don't cause internal injuries even if there is a goose egg. What to do depends on the details.