Good thing you didn't open it. Anerobic activity accounts for most of post-mortem decomposition; think of the Pharoahs. They were sealed up pretty airtight. Yet soft tissue and adipose tissue decomposed anway. It's better not to see loved ones, including pets, that way. Plus, the possibility of breathing in harmful bacteria from an opened coffin is too risky. You'd have to wear a surgical mask and gloves.
You'll see him again anyway....
