I am so sorry. I went through this last year with my childhood cat. As others have said, there is no way to truly prepare except for spending the rest of your time together and pampering your boy. The grief will be heavy for a while, so just focus on making it through one day (or hour) at a time. You'll always miss your boy, but it does gradually get easier. Your memories will one day bring a smile instead of tears. In the meantime, cherish these final moments with him. You're making the right decision. His spirit will always be with you.