Not sure. Two possibilities i can think of:

Bone fragments on the soft tissue of the forearm. If it is near the fracture this could possibly be. This can potentially be problematic as as the bone goes through a healing process laying doen more bone where it shouldn't (heterotopic calcification is the technical name).
bullet fragment that is irritAting the tissues
Localisd infection is another possibility.

Any of these need to be investigated

Trigger points - or otherwise known as myofascial pain. This can produce tender knotts and bumps in muscle bellies that are tender to touch and refer pain down the forearm. If it is this then they are relatively benign and will probably resolve as he gets stronger and with more mobility