Although this is technically 7 hours after the fact...
... with subdermal hematomas to the head you should not allow him to sleep immediately after the accident. You're looking for him passing out (from internal bleeding in his head). Plus other signs of trouble are vomiting and ... there's another sign but I can't recall exactly what it was now.
last september my younger daughter fell out of the stroller and landed on the back of her head. The 1st aid folks at Seaworld in Orlando were nice but offerred no adice other then to NOT LET HER SLEEP and head for an emergency room. We called my BIL who is a 7 board doc who teaches at one of Harvards' medical schools. He's a pediatric critical care board certified specialist - among 6 other specializations. And he suggested the period of observation was up to 96 hours with greater intensity in observation for the first hours and day. Oh yeah... you look for different dialation sizes (???) between the eyes or disorientation on time especially if you think your son passed out at all.
We opted NOT to go to an ER because they could not diagnose or counsel anything further then this ... and it would have cost us a few 100 or 1000.
If you need more info because of a protential crisis ... go to the emergency room or call 9-11. I can call my spouse when she lands and ask her what the other cardinal signs are for observation on a child following a head blow and large subdermal hematoma. She lands in about 90 minutes. I'll PM you any info to update this...