Never underestimate your child, they will blow you away every time.
You feel his teacher is expecting 8 year old behavior? I will be honest and say, I doubt this. She is expecting him to act the way she has witnessed him in the classroom acting at his best behavior.
She is encouraging him to quit his childish ways and reach his potential every time. I am sure she has seen it.
You and your husband need to step it up and remind your son that he is a big boy now. He will always be your child, but he needs to act like a kindergartener. This means "no whining".
You are doing good to tell him "use your words", "use your regular voice" etc. Now you can add, "go to your room and look for a regular voice" or "I cannot hear whining".
The main rule is to NEVER give into the whining. Do not give in even "this one time" just because you are tired and worn down. Let him realize you are serious. Figure out what seems to set off the whining.
Each time he asks for something without the whining, tell him you are proud of him. I liked how you asked for that cookie without whining. Thank you for not whining. (pick one)
Is it when he wants something and does not think you will allow him to have ?
Is it when he is tired?
When he is unsure he will be able to do the task by himself?
Is he afraid of failure?
Instead ask him. "Why are you whining? Sit down right here and think about what is making the whine come out." "When you figure it out, use your regualr vcoice so we can talk about it.
"Are you afraid, nervous, tired (pick one that you think may be the reason)?" "Do you need a minute to calm yourself down?"
Some children like to know exactly what to expect. "We are going to tball. Remember to listen to the coach and to cheer on the other players. " Remember everybody is just learning to play the game. Just do your best."
"We do not lay down during tball. Be a good teammate and sit up and watch your teammates.
"There is no winning or whining in tball, it is just everybody helping each other try their best"
At tball tell him, "I liked how you payed attention to 'Billy 'when he was at bat." "That was a good way to be a good teammate"
Your husband cannot tease his son. This just frustrates and humiliates his child. If your husband was being teased at work about the way he did his work, it would be called harassment. He needs to show respect you his son and be a person his son will trust.
I am sending you strength. Parenting is not for the faint of heart.