Oh, Sarah... I am so sorry. What a difficult loss, but I hope that the feeling of guilt will be short-lived. If he wasn't showing discomfort, whatever happened was probably sudden and very painless & without enough warning for you to do something, and I'm sure that his life with you was fantastic.
I hope that your other kitties are keeping you and Bryan company while you are grieving.
We lost our pet dog very, very suddenly years ago - it happened literally within 60 seconds and we never knew what happened, but she passed away while playing and having the time of her life. We should all be so lucky. Not that it made the loss any less sad, but I was glad to know that she didn't suffer.