To my knowledge, Bonnie does not have a canon voice. A lot of impressions or auditions that I've seen online give him a southern accent. That's okay, but I don't want you to be discouraged from using your own interpretation of the character. If you want my recommendation, all I can really say is to imagine what a male bunny would sound like (although you we're probably going to do that already...). Along with Chica, this is one of the roles that I urge you to have fun and experiment with!