Hello and thank you for your question.
Usually the bone cyst that occur in adults are benign in nature. However as he has history of cancer further testing may be necessary to evaluate whether it is cancerous or non cancerous cyst.
They may further test him by a needle biopsy test to evaluate if the cyst is cancerous or not.
If it is not cancerous it can be surgically removed. if it is cancerous than they may remove it surgically and give him radiation.
In my opinion usually the cyst is benign however in order to be on safe side test should be done quickly to determine if it is benign or malignant.
In both cases surgery may be required.
Please let me know if you have any further questions. Thank you.