It's not that it will never separate from his body but that he always has access to it, as in Garret can never ever be disarmed. Being that Proteus itself is a magical force, and it somewhat has a mind of it's own. What it does is if any part of it is separated from Garret's body all it takes is a thought from him to instantly call it back to him, as it shares a fraction of his soul. So when I say thrown like a javelin he literally makes a javelin and throws it, only for that weapon to melt and be recalled back to his hand. 

And as for how much armor he can have whilst still being armed, he can make a full suit of plate mail and still have enough for a medium sized shield and weapon. Or two medium sized weapons. One of Garret's signature battle styles is fully armored with two longswords.

Edit: Also, I elaborated further upon his race. I keep forgetting that I should probably explain everything that comes up about my own world.