Getting to a specialist through an HMO, if that's what your using, can sometimes be more painful than the injury itself. An MRI is usually a "referrable" issue in todays medical processes. That referral happens after the initial doctors "coarse of healing" is deemed not working.
I don't know the specifics of your sons injury, but, I do know that with most injuries that have strain/sprain symptoms an MRI is not the "next action". Because of the costs involved with an MRI, it takes some failed healing time from the initial diagnosis to get one scheduled.
If your the parent, you think this thought process sucks. If your an HMO member trying to keep costs down, you do not want every sprain/strain that comes into the clinic fast tracked to the MRI device.
I don't mean to be ngative, what I do suggest is be persistant and play within the medical game. Start by having the initial doctor set up a timeline of recovery before he refers you to a Sports Med specialist