There is a possibility of soft tissue injury in the leg which might be causing the pain and trouble while walking.
Was there any swelling after injury.
Next Steps
You can apply a crepe bandage for support, give him analgesic and anti inflammatory medicine, start ice fomentation, ask him to avoid standing and running for a long time.
Health Tips
If the problem still persists, then it would be better to go for one x ray of the leg and knee to rule out any bony injury.