Sorry to hear about your son. It is understandable that you are concerned about his brain lesion. Generally speaking, many people with a
cavernousHemangioma malformation are able to live absolutely normal, full lives (depending on the location and size which were not provided in the post.) Like your son, it's often found on accident or don't become noticed until much later in life like age 20-40. Most of the time they are clinically silent and you would never know they were there unless it bled (the annual chance of bleeding is less than 1%) or you happened to have a seizure,headache, or neurological deficit such as facial weakness (again, very much dependent on where this lesion is). Most of the time when it does bleed, it's just a very, very small amount and you wouldn't know it at all. The hemangioma tends to calcify (just its nature) and the blood becomes hemosiderin which is a blood byproduct (think:old blood) which leaves a typical dark ring on the MRI around the lesion. All that means is that there's been evidence of bleeding. As for the possibility of tumor, I would have to personally review the MRI to make an accurate comment. But yes, there are some tumors that calcify such as an oligodendroglioma which again, patients can live relatively normal full lives with.
There is no right answer as to how often you would scan this. 3months sounds appropriate and what is often recommended here, but there is nothing wrong with 6 weeks either. Choose a neurosugeon you trust and allow him/her to help guide you in the management of your son's condition. Good luck.