I can think of two reasons someone would say “aks” instead of “ask.”
He doesn’t know that the word is pronounced as “ask.”
He knows the proper pronunciation but chooses not to use it.
Regarding #1, how can anyone possibly not know how to pronounce this word (aside from deafness)? He must have heard it properly a million times on TV and radio.
This leaves #2. Why then would someone who otherwise pronounces words properly deliberately mispronounce this one?
In brief, WTF?
It’s properly pronounced “axe”. 
I have heard people using more or less standard English except for this one word. What’s up?
“Axe” is slightly easier to say. Pronunciations change for various reasons or style or no reason at all. Say this word: “Zebra.” I guarantee the way you said it isn’t at all the way anyone in the world said it 150 years ago.
“Axe” is also, by the way, the original pronunciation. “Ask” is a change.
What do you mean? My grandmother, an ex-English teacher, pronounced “water” as “worter,” “house” as “hoose,” and “coupon” as “koo-pon”. She was intelligent and literate. But her old-money-South dialect had these peculiarities.
So the third option is almost certainly the one:
3) He knows how some other folks pronounce it, but in his dialect, “ask” is pronounced “ax.” In other words, it’s the proper pronunciation in his dialect.
Around 1995 lots of people suddenly lost the ability to pronounce the letter “R”, The gansta wappa effewt for that and the pwarty cwowd help that along.
That would be correct.
Do you say cue-pon?
Or is from New Orleans.
Around NE Ohio, it is the quintessential Ebonics phrase: “I’m gonna axe my sister if she wants to see a movie Friday.” I hear it all the time.
Yeah, since you brought it up: There is no Q in “coupon,” so why do some people pronounce it that way?
How do you pronounce curious?
Qhy is how I pronounce “cu” relevant to how I pronounce “cou”?
But it’s not cu-, it’s cou-.
Cough, council, count, counter, coup, coupe, couple, couplet, coupon, courage, court, course, cousin . . . A cue- pronunciation for coupon would definitely be anomalous. And, in fact, I’ve never heard it, or been aware of its existence, until I read this thread.
I occasionally have a lapse in judgment and will pronounce “ask” as “aks,” and “water” as “warter.” I’d imagine it’s mostly due to people around me pronouncing it that way when I was growing up. I’m from Rockland, NY, if you’re curious.
How strange, as it’s the opposite situation for me. I grew up with “cue-pon”.
It is relevant because it addresses a point you yourself made, namely, that the absence of a Q gives you a hint about the pronunciation of coupon. There is evidently no Q in curious either.