I learned Russian for 1 year many years ago and from my point of view, you should teach your child the russian grammar, it will help a lot with the understanding of any other grammar he will encounter later in his life. I have learned 5 languages yet and I hope my son will be interested in learning some of them. We are french speakers and are living in an english environment and even if we don't speak english with my son, he is picking up some words from cartoons on TV and people talking to him. He is 3 years old. Teach him using the syllable method, it is the best method for any language to begin reading.