My mom kicked me out when I was 17 but it had nothing to do with my sexuality. There were other factors in play and she told me and my twin brother that It would be best if we weren't home anymore. (1gay 1straight) It was hard and I hated her so bad for several years.
I think it is a parents responsibility to do everything they can to give their child a good, safe, loving home, to protect them and prepare them for the future which included their education. I know that not every parent has the money to pay for college but there is more to it then just being a checkbook! Helping with student loans, scholarship applications, room/board, books, finding time to study vs have to work. It's a lot and I remember being all alone and not having anyone help me with any of those things.
I think Billy should wait to tell them. When he's older and more mature and ready to handle their prejudice. Then it would be their decision if they lose the wonderful child that they have, one that they should be super proud of, not disowning him.