You have some great responses here. I'm a right handed mom of 2 Leftys (ages 20 and 9) and having your child trace letters as well as placing your right hand over his left, you can guide him when he is having difficulty or attempting to master a new letter. The main issue that left handed children have when learing to write their letters is that often their hand covers what they have written.
The way I taught my left handed children to tie their shoes was to just sit beside them, each of us having a shoe directly in front of us, and just had them mimic my motions as I tied the shoelace. This worked out VERY well and only took each one of them a few minutes to learn. Your son will naturally do it left handed, but no need for mirrors, although that could work too.
If you approach teaching left handed children how to do things in a relaxed manner, not even worrying about the fact that he prefers his left hand, he will find it quite easy to learn and adapt to everything he needs to do. Just make it a 'non-issue'.