[cypher]

I wouldn't use the brush, for a tiny robo a brush approaching him must be quite intimidating & I would totally expect him to either attack it or just run away!
You can try just burying your fingers in the substrate with a treat on your palm if your worried about bites, I know it can be hard but if he does bite try not to react or make sudden movements just gently & slowly move your hand away.
Have you tried doing the tissue trick (putting some toilet tissue up your sleeve for a few hours then shredding it up & putting it in the cage) so he can get used to your scent in the cage?
It's often just a case of taking time & being patient with them, most will grow out of biting with time, with some it's a bit of a territorial thing & they just don't like hands invading their space in which case it's better to do taming in a play bin or playpen.