Offering two rodents about the size around as the thickest part of the animals body, once a week, or every other week, should be sufficient.
As far as type of snake, I would definitely go with black rat. Although genetic background is near impossible to tell, without some sort of extreme DNA testing. I see no reason to believe otherwise.
There are many basic care sheets on keeping ratsnakes online, including, I believe this website.

BTW Beautiful animal.