I'm so sorry for your loss:(. I've just recently lost one of my gliders that was around 6 yrs old. Unknown cause, no signs of illness, or trauma (the way I found her was a mystery). Sometimes we have to accept the not knowing why. It's hard enough with just greiving the loss, you want to understand why, but there's not always an answer. I'm still struggling with that myself, just over a month later.
The best you can do is remember the joy he brought to you, and know you brought him a better quality of life, where he was truly loved. He knew that.