Prompting Speech for Autism

What is the best way to prompt speech in a four-year-old, otherwise high-functioning-autism child?  He has:  abilites to make both frontal sounds (b,m) as well as gutteral sounds (h,j); close eye-contact to my mouth for clues as to how to make a sound (rounds his lips like mine to say "o"); ability to hum, make loud "eeek" sounds as well as soft "oh" sounds; and has no hearing or motor difficulties.  What I cannot get him to do is respond to prompts in the same way over a period of weeks. That is, if I use a reward system, he can say "b", "m", "o", "i", "h", "han" for "hand"- - individual sounds that we have learned- - but after about one-two weeks he stops responding to the rewards. I've also tried giving him tokens, praising his responses, letting him do something fun after "talk time", using a mirror for him to see his mouth while speaking, placing his hand on my lips to feel the air/vibration on my neck, trying to get him to sing words, and even attempted some assisted articulation methods.  He doesn't like putting things in his mouth including fingers.  It seems like he can say a word, but then will file it away and refuse to use it again.  This is frustrating in that I feel speech is there, but not making progress.  Do you have suggestions how to stimulate interest and motivation for speech.  He would starve if asked to say "I" in order to get his food.  It seems he does not care about talking.  If it wasn't for the diagnosis and his mild stimming behavior, I wouldn't have known he was on the spectrum. Thank you for your response.