Hi, krmilmont:
Although no parent wants to have their child go through the ordeals of having an AN tumor, surgery and radiation, you and your son seem to have weathered the storm. Well, almost.
I won't pretend to have anything close to a definitive answer to your original question. Only a therapist who spends time with your son might help you both find an answer. While the 'irrational decisions' you refer to could be attributed to the tumor and all the emotional issues AN patients often suffer during the pre and post diagnosis stage as well as after surgery and even radiation, as a teen, he could also be simply 'acting out' based on other issues, as teenagers are known to do. His behavior might also stem from normal teenage mistakes (born of inexperience) combined with the extra stress of the AN 'baggage'. I'm not too sure if there are any purely physical brain-related issues involved with an acoustic neuroma tumor - even a large one - that directly or indirectly impact decision-making. However, I'm not a physician, although my wife and I did raise a now-adult son who had a relatively drama-free adolescence.
I didn't become irrational but I did become grouchy after my diagnosis and especially after my surgery. I simply hated being 'watched' (by my infinitely patient spouse) and being considered 'sick', when I felt pretty good and recovered quickly. However, I have almost 50 years on your son, so I doubt my experience is germane to his situation. However, a few of the parental posters on this forum have 18-year-old children that were diagnosed with large AN tumors (and suffered many complications - but recovered) who might respond to your question with more relative experiences. Lets see.
I hope your son continues to recover and that the days of his making irrational decisions are now finished.
Jim