If your collection grows I have two pieces of advice...
1 - sort your pieces by element type, do not sort by colour.
2 - point 1 will be difficult for a 5 year old to grasp, and you don't want to scare him off with boring talk about how to sort lego. So I recommend having a shoebox sized container of random mixed Lego that your son can dive into at any time, without the risk of him causing havoc in the main, efficiently sorted and stored collection.
Having said that my boy of similar age often ignores everything I say and I've resigned myself to a good 5 years of non-stop sorting up after a tornado.