Head Start?

Trying to give my four-year-old son a head start, I am currently trying to teach him how to read. I made up flash cards with "sight words" on them and together, we sat down at the dinner table to try to memorize them. Mason got stuck on the word "of". Sounding it out, he kept pronouncing it like "off".

I explained to him that the word "off" had two fs, the word "of" just one. Finally, we got it and moved on to the next word which was "as". I explained to him that like the word "of", "as" was pronounced differently than it sounded out. It was pronounced "az". He then explained to me that if you added an extra "s" to it, it made the word "ass".

What could I do? He was right. The only course of action left for me was to ask him to use it in sentance.