Welcome to the site,
I understand the horror of finding out your son has these issues. Our son had Epilepsy, which involved a lot of medication and eventually two brain surgeries. There is nothing fun about watching your kids go through something like this.
Initially, your best source of info might be the local DR. Take excellent notes: each of you should take a journal with you to every appointment. Write all of your questions down prior to the Appointment as you think of them. Make sure each of you have a copy of all of the questions. As you ask them, jot notes down of the answers. If the DR steps out, immediately start comparing notes to insure you each heard the same thing. If not, ask for clarification. Right after each appt, find a quiet spot and compare notes again.
Keep a copy of any test results you are given in your "book". If you don't get a copy of the report, ASK for one. As you are referred to specialists, you'd be amazed how often your medical records are at hand easily for future DRs to review. We've had Drs take what we had in OUR book, run a copy, to put in their records rather than trying to chase down the official one that is lost somewhere in medical never never land.
In a journal, log the date, Dr name, address, and phone of each appt. Keep a log of medications prescribed and any complications you notice. Same with things like MRIs, CT scans, etc. Right now, you can remember that you had an appt last week. In six months, you won't, but your journal will contain the right info. Almost all scans are digital now, so getting a copy of the disk is cheap, if not free. (With wife's AVM, occasionally, we sometimes paid about $100 to get a copy of the X Ray-like scans to carry with us.)
Since your son might have both a malformation AND cancer, right off the bat you will be dealing with two specialists. Up front, make sure they will work with each other. We phrased it like "we want you on OUR team, but it will involve close coordination with Dr X." Usually it worked fine. When you get to specialists, they sometime have egos that rival their salary!
We've had more than one doctor see our books, us taking notes, having copies of reports right at hand, comment to us how refreshing it was to see patients this organized! Things like with all our questions written down, we could ask them quickly. Not like we'd sit and ponder, wasting the DR's time.
Both with my wife's AVM, which was treated on the west coast, and my son's epilepsy, which was treated in St Paul, when seeking a local neuro, we explained in appointment #1 that the specialist in west coast and St Paul were the lead dog and would be calling the shots. We wanted our local guy in the loop, he would get a copy of any correspondence or medical report. And we welcomed his input to our specialists. Any testing that needed to be done, and could be done locally, our specialists worked it through our local neuro. Worked pretty well, mostly.
While most if not all the medical folks you run into will care greatly about your son, YOU TWO have the most interest in seeing this work out well, so don't be afraid to be a part of his treatment team.
Best wishes to you,