Decomposing is typically only in reference to tissues/organs/soft matter. Things that bacteria, insects, and animals can eat (thus causing the decomposition). This happens rather quickly.

Bone can't be eaten, so when all that is left is the Skeleton there is no more decoposing. As long as they aren't crushed or destroyed through other means, bones will last virtually forever.

I assume he was identified by dental records or something similar (healed broken bones, etc).