Our daughter mostly spoke English at home and watched English TV.. once she started going to a local kindergarten that all changed and her Chinese improved way faster than English, to the point that her Chinese is now better than her English. So I would first recommend getting your son to a local kindergarten just as soon as you can, and to get dad using the Chinese. Being able to socialize, fit in, and mostly communicating like a local for when he starts going to school is going to be the most important thing so he can make friends.
I was worried about our daughter, as the only mix baby in the school along with suffering from real bad eczema, I too thought she would have a target on her back.. I should not have worried, at least regarding the bits I was thinking would be problems.. kids often don't see what we see..
Yes there was a little name calling (using qu-yi-zi instead of qu-zi-yi but all kids got a bit of that first day), the worst was stuff was a couple kids telling her she had 'dirty' hands so she would go wash them, then when she came back telling her they are still dirty (mum is from the south), she quickly learned who to avoid, but nothing too serious that she could not handle, I think all kids go through a little of that, and it teaches good social skills. Now in grade 6 her and her group of friends are like peas in a pod.. they're inseparable.
Having a western background, I also assume you will not be pushing your son to get 100% on all tests or be top of the class, rather to just try his best, enjoy it and possibly learn something. As Andrew eluded to that attitude in itself strangely seems to help.. many of the Taiwanese kids are expected and struggle to do well with lots of pressure from their parents.. so they are competitive, if he is not out to beat them but rather a friend, they will be more accepting of him, if they can practice English spelling together.. all the more useful.