Hey, I am Marcel and I was in a similar situation half a year ago. I got my diagnosis in October 2020.
I am 19, similar to your son.I too, had a cerebellar AVM with a similar size that did not yet rupture but showed symptoms.
I am sorry for the huge chunk that I wrote below, I hope it helps!
I am not a doctor, nor a certified professional in this particular regard, but I have dug deep into the literature and had a similar starting position that your son is in now.
General information I gathered: (out of memory, I do not have all studies at hand, but I can send you some to verify if my statements are accurate)
When it comes to risks regarding AVMs in the brain, many studies seperate the ācerebrumā (upper, big brain) and the āPosterior Fossaā (brainstem & cerebellum) [<- what we are concerened about]
The annual risk of a bleed is often cited in articles to be 2-3 or 1-4% and that is true, but mostly for AVMs in the cerebrum, not in the P.Fā¦ Bleeding in the cerebellum or brainstem is about twice as likely with some citing 4-6% annualy. Not only that, but the chance of finding a pfAVM that has not yet ruptured (as it is the case with your son and me) can be as low as 8-20%, the rest gets found with a dangerous hemorrhage.
What I am trying to get across is that you are (as was I) in a very privileged situation. The odds of a hemorrage are high, but you are in a unique situation to get ahead of this desease. Intervention regarding unruptured Cerebral AVMs is under debate, but most neurosurgeons agree that intervention of posterior fossa AVMs (especially cerebellar) tends to have a good success rate with good outcomes and is generally standard practice. The high chance of hemorrhage is also a key factor that validates agressive treatment. But I may not need to tell you all this, since it seems that both of you are in favour of surgery, just the timing is giving you your dilemma.
Brainsurgery can be scary, but I would hope that you dont look at it as a hurdle to his success, but rather a chance. Cerebellum can recover blisteringly fast and it may be weeks, and for some months, to regain almost 100%. It is almost certain that your son will lose time as a result, but he will gain health simultaniously and this damn desease will not bother him after the recovery is complete. Due to the pandemic, my most recent chance to operate was in late February. In my experience the most time that I lost was the time leading up to the operation, as my symptoms grew stronger and I could not excercise, while living under the stress of the diagnosis. I wanted it done as soon as possible. I cannot give you advice regarding the things your son will potentially miss out on, as I am from germany and very unfamiliar with your system and institutions. What I can say tho, is that I have also lost oportunities because of my operation, but I have encoutered a lot of goodwill and obligingness when I told them the reason why I missed out.
I also want to adress questiones you mentioned, I will give them a try but take them with a grain of salt:
Q: Will communication/reasoning get compromised?
A: Probably not. The Cerebellum is mainly responsible for balance and coordination of movement. There is a part in the deep cerebellum that controls facial muscles and it could happen that a side might sag and speech may even be slurred for a short while (not permanant with rehabilitation, no worries). That should not really interfere with communication and especially higher brain functions. There is some inconclusive speculation that the cerebellum has influence of concentration, however these differences are hard to measure. In other words, he will most probably not loose any relevant higher brain function nor cognitive ability. He will not lose his intelligence.
Possible symptoms however can be:
Gait Ataxia (balance problems while walking [like a drunk person] but very treatable and almost cureable with rehab, and good results after a short period of time (4-10 weeks [my experience, and I had a big emergency operation]) especially in his (and my) age.
Nystagmus (eyes rotate from on side to another, to counter a rotation that does not exist [awkward, but goes away in a couple of days])
dizziness
vertigo
All these are generally temporary or can be mitigated very efficiently.
You posed additional questiones:
Q: They are good and we trust them. But can we wait?
A: You can of course, but if they are good and you trust them, there is not much reason to wait 
And if they say that its a good spot that is a VERY good sign that they are confident about a good outcome.
Q: Medical management, what is it?
A: Well āManagementā in AVM treatment usually means wait and watch what happens. (Occasional MRI scans to see any changes.) But while they can see aneurisms and bloating vessels, predicting a rupture is almost impossible, and that is bad, since a bleed in the cerebellum can turn this convenient AVM into a difficult and life-threatening one.
I really dont want to scare or push you into a choice, but I really need to emphesize, that your son is the perfect patient to operate on right now, however, should he get a bleed, wich could happen anytime. His chance at a good outcome or mere survival will be drastically deminished. Only about half survive that sort of hemorrhage.
He is a busy and obviously very intelligent man, I would be surprised if his ventues will become less in the coming years. You can postpone his operation to give him time and space to pursue his activities and hobbies, and that is fine, but the āright timeā may never come. He has a bright future ahead and an operation can clear his path. Deciding to postpone the operation in an effort to save his carrer and social life is totally understandable and under the right circumstances reasonable, but please be aware, that he himself is at stake.
Part of what can make this desease bizzar, is that people are at greatest risk, when the sun shines the brightest and their health seems pristine, but when I got out of the operating room and I looked the worst and weakest, I cried because that was the exact moment that I was truely cured for the first time in my life.
, but I would recommend you look at it all from a worst case scenario point of view. We all hope/wish/pray it never happens, but āWhat ifā¦?ā If you plan for the āWorst case scenarioā and it doesnāt occur, thatās a bonus. But if you donāt have a plan and the āWorst case scenarioā occurs it can REALLY knock you off your feet.
