I read it as a young teen (so early 80s). It was (I think) a four book series. It starts with an attack on the home-world of the 'Interplanetary Police' - there was a name for them, but if I could remember it I'd probably have the title :) - with a radiation/poison that ate into the bones. All were killed.
The protagonist was rescued by a kindly old wizard (sorry, scientist), had his bones replaced with an unbreakable material (to remove the poison) and given an ally / side-kick of a flying batlike (BIG bat) alien who may also have been telepathic (bit fuzzy on that) and sent after the baddies.
I think there were 4 in the series.