Phonics. It's what he'll do at school. Letter names will confuse him when he starts. Google phonics and you'll find so much info. Try jolly phonics to start. 'Mr Thorne' has videos on YouTube and will show you how to say the sounds properly eg sssssss not suh, and mmmmmmm not much. And yes you definitely don't need to 'teach' a nearly 2 year old just say the sound he picks up correctly. Good luck!