We're going to hongkong on mar 18 to 23, 2015 with my 4y/o son.

It is most likely to be cloudy during that period with the average temperature of around 20C (68F). Prepare long john, overcoat and relatively thick jumpsuits for your son. For you, a leather jacket, thick long-sleeved shirt and jeans can be ok. According to the previous weather data, light rain might happen then, so pack simple rain gear as well.