It sounds like your son is getting good care. I am not familiar with the type of lesion your son may have, but annual mri's will probably be recommended, to monitor any growth or changes. If your son has no uncomfortable symptoms associated with this, I don't see any reason to expect a recommendation for surgery. I have read that many people probably have brain lesions their whole lives, and never know, unless they have an mri for some reason. I believe lesions such as dermoid cysts can sometimes disappear on their own.
The "wait and see" approach can be frustrating, but it is a common course of action (inaction!).
Precocious puberty can be caused by hormonal disruptions from many sources. For example, soy foods are notorious for causing thyroid disorders, precocious puberty, and other endocrine disorders. Unfortunately, it's a rare doctor who looks for the primary cause of the lesions they treat, in my experience.
Keep up with the diagnostic tests (serum,urine etc.). Something may show up outside the normal range within the next year.
I wish you and your son the best,