I went and took a look at a few photos of Tucker saddles, and it looks like there is a definite possiblity that the saddle is putting your husband in a chair seat which would make posting difficult.
What I would do in your situation is:
Purchase book, Centered Riding by Sally Swift. Read.
Get Hubby some lessons. English saddle of some sort, just flat work, not any jumping. Explain to instructor what he needs to learn - posting the trot. It is the instructor’s job to see that Hubby builds a correct seat. Hubby’s job is to learn how that seat feels to his body. You need to watch the lessons too, so that you see how he looks in a correct seat. A correct seat is beautifully illustrated in Centered Riding, as well.
Then, after several lessons, he rides in the Tucker again. You need to look at how he is sitting. If his feet are in front of his hips, he is in a chair seat, and it is the saddle putting him there, because at this point he should know better. He needs to ride at the trot, and see how he can post. Now that he has had some lessons, he may find it easier. Or, he may want a different saddle…
Without seeing him riding, it is hard for us to say what the problem really is. He may simply have to build some new muscles for posting to be comfortable. The lessons will help there.
One last thing to consider. Some horses make you post wonderfully and it is the most natural thing in the world, easy peasy! And then there are the others - posting is hard work on them. So that is another thing to consider. Lessons on a different horse might help him know something there, too.