[mst]

Converse.

Asking questions like "what flavour of juice do you want?" will encourage a reply of "apple" or "orange" rather than a monosyllabic yes/no.

Being ok with whatever the closest approximation of that word you're going to get is and then repeating back the correct pronunciation (as mentioned in another comment) makes this a lot less stressful than it sounds.

If the kid's mother is the primary source of unconditional cuddles, considering becoming the primary source of respect. When I was in a similar sounding situation, the desire to be "daddy's big boy" was a powerful motivating factor for getting him to stretch his abilities.

Also, though ... the fact you care enough to have started this at all suggests that the parenting circuits have kicked in (or ... whatever you want to call it; let's just say I know the "oh shit, I appear to feel like a parent ... ok, wow, this is amazing also I AM SCARED AS HELL OF DOING IT WRONG" feeling) and that combined with a little common sense will go a LONG way.

If you've already "been told his rate of development has been remarkable since (you) became involved" ... you may be micro-optimising something that's already proceeding pretty efficiently already.

[Tcepsa]

I've also seen suggestions[citation needed, sorry] that when offering kids a choice, narrowing it down to just two or three things can help them start getting used to making decisions without being completely overwhelmed by the options (e.g. if you're at the grocery store and you decide to give him the opportunity for some input into the purchases, ask, "do you want to get apple or orange juice?" rather than taking him to the juice aisle and saying "what kind of juice do you want?")