Okay, maybe the rigid armor would be a big factor in killing him, so maybe the armor should be a more flexible material.

 

What about a foam rubber suit?

Have you ever seen the Mythbuster episode where they're trying to break a beer bottle against a motorcycle helmet? They can't do it, because the helmet is designed to absorb the shock and spread the energy across itself so well that it won't even shatter glass. If you threw a beer bottle against that rigid, inch-thick steel body armor, it would easily break.

 

For the bike rider, it would be all about collision and preventing as much shock as possible, spreading it more evenly across a wider surface. What you've got now is more like tank plating to stop bullets.

 

To kill a properly armored hoverbike rider, who probably has some fire protection as well, you're going to need something that doesn't attack him where he's strong, especially when it needs to be hard to detect or trace. I'm starting to like the poison or electrocution angles. Something that's going to make him lose control and fall off that bike at his top speed. Physics does the rest.