It's tough for a young man in this situation. He is naturally craving independence. Your hubby still wants control. It becomes a pack mentality thing.
You are right to encourage your husband to build and maintain trust and respect. HE wants respect from your son but your SON wants respect from him, too. With males, it's all about respect.
As long as your husband makes this a control issue, your son will never acquiesce. No human being wants to feel controlled, being under someone else's thumb.
Rules are made to be broken. Guidelines. Courtesies. Living agreeably together because you're in a communal setting not to mention you're still a family. Those are better. There has to be reasons for rules if your son is going to cooperate. Rules that might protect him, keep him safe, okay. Rules just because your husband wants him home at a certain time, etc.? Phooey.
He'd better be able to explain his reasons for his rules, especially since your son is 19 and of legal age.
Sit down with them and encourage your husband to really listen and hear your son's desires. Treat him as the young adult he is.
Talk with your son alone. Explain that you understand that he wants, needs and deserves freedom and independence. But he needs to think like an adult. If he wants or needs to live with you still, he needs to be considerate of his stepfather's wishes, too. He's paying for that roof over his head, his meals,etc. Is what his stepfather is asking too much? He should be able to keep his own private room any way he likes but keep communal living spaces tidy--at least clean up after himself. He would have to do that if he lived in a dorm or if he rented a room somewhere.
He can always get a job and rent his own place--like an adult. Neither of you will be able to change your husband. It really is a take-it-or-leave-it situation for him. Remind him of all the wonderful things your husband did for him while he was growing up. Tell your son that if he wants respect from his stepfather, he needs to offer respect first. Who cares if he isn't his biological father? He obviously deserves more respect that the man who hasn't been there for him at all. Everyone deserves respect, especially the man who raised him and loved him.
Talk with your husband alone. Explain to him that you understand that it isn't easy living with a young male who wants to make his own rules. That your husband does have the right to set his own house rules. But let him know that a little flexibility is good for the soul. Point out your son's positives. Thank your husband for helping to raise such a fine young man.
But the time for rules is over. The time for mutual trust and respect is here. They can have a wonderful relationship if your husband will learn to treat your son like the young man he is.
Remind your husband about how he felt as a young man, wanting to be his own person without anyone telling him what to do. Remind him that young males go through these volatile periods trying to find themselves. It takes patience and space on behalf of the parents. My son went through this phase and now he is just wonderful.
You can have a wonderful new relationship within the family if both sides will remember to feel thankful towards each other and then do the small things to make each other happy. It doesn't take much to offer respect and appreciation.
Tell them not to throw the baby out with the bathwater. Focus on what they do like about each other, what they are grateful for. Good luck.