If it was full dislocation the pain level would generally be a lot higher - it does sometimes pop back in by itself but may hurt for a few days afterwards. I have issues with radial patellar dislocation in my right knee as a result of an old sporting injury, the last time it happened when I simply stepped off a kerb to cross the road. Wound up in a hip to knee brace for a week and 6 months of physio to try and avoid the need for surgery.

Definitely get it checked out, the doc should possibly send him for x-ray or ultrasound to see if there is any ligament damage and may recommend strapping for sports etc.