he will , and it will be so worth the wait X
I do understand what you mean about the star chart and not wanting to be mean if he is struggling.
You could always not ask him to say words , and yet still have a talking sticker chart, and when you feel he has suddenly come out with a word, like he is using daddy, lots of praise and over to the chart to stick on a sticker, you don't have to sell the chart to him, the reward will be enougth , he does sound like he is good understanding things, so i bet he makes the connection between words and suddenly getting stickers very quickly, with no negative or overdoing it outcome, he can choose his own words this way, when ready
keep up your good work tho, I imagine it is more draining on you to be talking him through it all