Thanks for placing your query here.
I can appreciate your concerns about your son.
Handlebar injuries are very common in children.
There are two possibilities of the painful twin swellings under the skin.
(blood collection under the skin )
2/- Abdominal wall hernia
Getting a Pediatric surgeon's opinion & Ultrasound
of abdomen will clarify the matter.
Hope I have answered your query.
If you have any further questions I will be happy to help.