I am sorry to hear about your father.
I have heard of a similar case - my cousin has an inoperable cousin in his thalamus which is just below the corpus callosum.
If you read up on any site - they give ranges. the reason why is there are so many factors at play here besides the tumor. How healthy was your father before the tumor, his habits (smoking, drinking etc) and his attitude can play a role in survival.
Grade II is still a low grade so it should be slow growing.
Radio-surgery is a form of radiation. It may or may not be an option as it depends on the type of tumor and whether it responds to that form of radiation. As well, with such a large lesion, you have to consider side effects.