Alliegid.
Sorry, I have just seen your post.
Have you asked me about this before?........I recollect some postings......probably not you.
Monocystic fibrous dysplasia is NOT cancerous.
The cause is unknown, but thought to be genetic............pretty rare condition.
Your son has it in his clavicle and as you say it is getting larger over the past four yours. it may continue to get larger, or it may suddenly stop growing.
It will never go away but it will never kill your son.
If in the future he develops pain, then surgery is indicated.
Surgery is successful and involves excision of the affected area with bone grafting, usually from the iliac crest.
I am sorry that I cannot be more specific, but that is all I know about it.
Do not hesitate to get back to me for other queries that I have not addressed.