Well one thing he is going to have to do is to become more assertive with the children. You can assist him in this. First, when you go throughout the day make note of things that the children did and ways that you would correct them (but do not correct them, just make a note). Do not tell your husband what you observed or the corrective action in front of the children, but discuss it in private.
When you discuss it in private tell him the behaviors that you noticed and come up with ways that the children could be disciplined or have consequences. You are doing this part, so that your husband can learn what behavior is inappropriate. When you point out specific behaviors or things that they said, and then give him the consequence on a regular basis he should begin to recognize it on his own and start to implement punishment on his own.
The next day or next time that the children talk back or say something disrespectful to him, have your husband implement the punishment that you two discussed the day before. The reason for this, is because you want your husband to begin doing the punishment and that it does not fall back on you. You also do not want to correct them in front of your husband or tell your husband to correct the children while the children are standing there. The reasoning being is because they will feel like their dad is changing because of you or that you are telling their dad what to do, and they may act out more.
If throughout he day there is a missed opportunity that he could have disciplined the children, take a note of it and have a discussion about it once again in private and what consequences you both feel should be appropriate.
So basically your are transferring or training how to discipline the children, but you want him to do it. I think that as their biological parent if they begin to see him implementing some of the same punishment that you are giving them then is will decrease their disrespectful behavior because it will be seen as two parents working together.