The little rubber thing is nice when they're teething but not biting too hard. Even if he is chewing on it, he is rubbing his gums and teeth against the 'nubs' on it and potentially rubbing away sugars.
Our son loves toothbrushes and he found this love just by imitating me. When he was younger, I was such a worry-wart that I even took him into the bathroom with me in the morning when I would brush my teeth (because he was just starting to walk and I wanted him contained while I finished getting ready). Me brushing my teeth became this silly game that held his attention and made him laugh and laugh. I'd open my mouth so he could see the foam, make silly faces, etc. Pretty soon he started snatching my toothbrush from me and tried shoving it in his mouth. Next time we went to the grocery store, I bought a new toothbrush for me and a new baby toothbrush for him and now we 'brush' our teeth together. Of course his doesn't have any toothpaste on it (18 months), but he knows what the toothbrush is for.
Try brushing your teeth in front of him to see if he catches on and tries to imitate you. Buy several toothbrushes in case he tries to brush his hair with it or drops it on the floor of the bathroom. Eventually he'll get the hang of it, and when it is time to really start brushing the teeth it might not be a chore!
Good luck - it's really funny and cute when they imitate this!