I never intended to teach N subtraction this early, not until he somewhat masters the additions. But i accidentally found out that he was actually learning it from our daily ongoings.
It happened one day, in my desperate attempt to make N eat faster, i promised to 'free' him from the table after he finish 10 more spoonful of his food. As i fed him the 10th spoon, i heard him asking me '9 more?', and on the 9th spoon, he asked '8 more?' and so on until the last spoonful, he would declare 'no more!'. This went on for many weeks until it struck me that there in my face was the perfect opportunity to introduce to him subtraction!
And so, i began to teach him subtraction...during our next meal. Yes, i decided to go without any visual aids and go verbal and be informal. So as he ate his 10th spoon, i would simply ask him 'how many more?' and he would reply '9 more'. And i would say so '10 minus 1 equals 9!' and so on. It didn't take long after he got the hang of Subtraction. Subsequently, i wrote the equations on the some cards so he can have a visual on what subtraction equations look like.
Since then, he is able to do any subtraction equation (i.e. X minus 1) even for higher numbers like '100 minus 1' or '300 minus 1'. I think what made it easy to teach was that he could count backwards prior to this and he understood that minus 1 means going down a number by one.