we started brushing our son's teeth at around 6 months of age as he had about 9 teeth then. I just let him watch me doing mine a few times which seem to interest him greatly. It's also easier we found if you buy 2 toothbrushes. One for him & one for you. That way he'll think it's great fun pretending to brush your's at the same time. Our son took to it really easily maybe because he was so young and now at nearly 21 months old he loves brushing and even does some of it himself. Another good thing is to do it in front of a mirror so he can see what's going on......he'll love that!!! Also if he has a favourite teddy or the likes, let him "brush" teddy's teeth!!!! Some days may be harder than others especially if your little one is cutting more teeth and their mouth is sore. Just try to make it "fun" at first and he should be ok.
Hope this helps,