Find That Book

Hello all,

When I was in single digits as a child (which was a LONG time ago as I am 35), I read a book in my school library which was about a hero on another planet (I believe it may have been Mars but I can't be certain). I can not remember this hero's name. He was a martial artist and I believe he had some sort of an animal companion.

I believe he was human, but was hiding from (and often getting into fights with) alien soldiers. It was a central detail that the main thing this character had going for him was his super human abilities with martial arts (but a made up/alien variety). Somewhere amongst the early chapters of the book, there were a few fight scenes in underground tunnels.

On the old copy of the book I was reading there was an illustration of some sort of hollow Iron Suit; maybe some sort of armor. I believe the name of the book was even some sort of reference to an iron suit, iron man, red suit, martian suit, hollow man etc. Problem is that whenever I search anything similar I just find 100's of Marvel references, to which this book was unconnected.

I would love to find out what this book was called as I loved it as a child and would even quite like to pass it on to my kids, if only I could remember what is was called. Any help would be much appreciated!

In summary:
About 30 years ago.
Childrens SciFi,
Strong emphasis on Martial Arts (but made up),
Another planet,
Underground Tunnels
Possibly Aliens, an alien army or alien soldiers,
References to some sort of suit of armor, possibly made of Iron.

Many thanks!