The only way I can think to discribe it is to call it a distorted short vowel a sound? It starts out sound like short vowel a, my mouth is in the same position but the n and m change it? So it must be a regional thing? If it is how should I go about teaching it? My son can sound out the words properly but then hasn't a clue what the word is even though he knows it because it sounds wrong to him (and me!).

My son and I have been working through The Ordinary Parents Guide to Teaching Reading and I am at a loss at how to explain to him why the short vowel a sound changes when it precedes the letters n and m. For example when you say the words man or ham you do not use the short vowel a sound that you use in the word mat. Am I crazy? I've looked all over the Internet and the book hasn't mentioned anything. Help please!