Thanks for writing to HCM,
For a hair-line fracture
a 4 week cast is usually sufficient followed by regular physiotherapy
over a period of time which will improve the range as well as strength of weakened muscles, stretching exercises will definitely improve the tight soft tissues.
with the above mentioned therapy the pain should gradually reduce.
If there would scaphiod fracture the first x-ray would have shown that, and ligament injury
can be suspected only after knowing the exact mechanism of injury but if you want to rule it out completely an MRI
scan will give you a detailed picture, and for this You can definitely consult your orthopaedician again for a review.
I would suggest you to take suggestions from your ortho as well as your physio before participating in any sports activity.
i hope this helps
wish your son a speedy recovery