Children have quite astonishing powers of linguistic acquisition - in other circumstances your son could be equally fluent in two or even three languages. Bear in mind this is your son, so you are a major source of his exposure to language, particularly in these very early years. To a considerable extent, you determine what level of language he becomes comfortable with - almost by definition, it's what you expose him to.
So don't set yourself an unrealistic task - for example, by trying to learn and reproduce a subset of grammatical constructions and vocabulary deemed by someone else to be appropriate for an average child of your son's age. You simply won't be able to maintain this consistently, and you'll end up constraining your own natural powers of expression.
It shouldn't be difficult to avoid words and constructions that you know perfectly well are excessively unusual or complex. But you should concentrate more on making sure the subject matters you write about are those your son is keen to engage with. And try to encourage his powers of imagination to develop; you want him motivated to read for himself, and perhaps to find even more depths in a tale when he reads alone than when you read it aloud to him originally.
Another little trick to try is introducing family/local references into a story. Even to the extent of having your fictional characters mention your own son as someone they've heard of. Artistic license is a useful tool for making sure you capture both the attention and the imagination of your reader!