Re: reassurance for the newbie
I can't really help you with this, as I have no knowledge of this particular type of mass. But I did want to encourage you to be assertive with your son's doctors. Let them know that sometimes it might be easier if they tell you all the things it COULD be than if they leave you to worry about the worst thing it could be.
It should probably take a couple of business days for the doctor to receive the report from the MRI. If you don't hear from him after 3-4 business days, call his office and just remind them gently that you are on pins and needles waiting for some answers, so can they have the doctor contact you as soon as possible. I have found that being very nice to both the doctors and the office staff, while still being fairly assertive, works small wonders.
But the doctor is absolutely right that this is a step-by-step process and one thing doctors hate to do is speculate without having enough information to be definitive. So sometimes the best question is simply, "and what's the next step after this?"
Tough to wait out a long holiday weekend, though. My thoughts are with you and your son.