First, you need to remind your husband that your stepson is a human being. And try as we might, we can't always get other humans to make the choices we want them to. We can teach, advise and persuade until we are blue in the face, but ultimately your stepson's decision on how to act is his to make. What he decides to do is not necessarily an indicator that your husband is a bad parent.
As humans we tend to seek validation for our actions in the results they produce. We feel proud of ourselves when we can see that the work we did paid off. I think we have all felt the satisfaction of seeing practice and hard work pay off, of practicing and seeing results.
As parents, we look to our children as the "results" that validate that what we are doing is right. For better or worse, your husband isn't the only person influencing your stepson. Your stepson gets influenced by his dad, his mom, any siblings, his friends, his school mates, his own personality, his values and beliefs, media and a whole host of other things. Unlike practicing a piano where one person's effort directly affects the results, people are a complicated mix of hundreds of battling influences. Since your husband isn't the only influence in play here, he can't be held solely responsible for his son's successes or his failures.
Also, reassure your husband that holding children accountable is not wrong. It is the only way they learn responsibility. No one has become a successful, hard working, responsible adult without learning that actions have consequences. Your husband was trying to teach that important lesson. Your stepson found it easier to bail out on learning something hard, but worthwhile. That doesn't make your husband a bad parent. In fact, he should be commended for trying to make your stepson a better person despite all the opposition.
Your husband is probably also battling feelings of "what more could I have done for my son?". The fact that he has those feelings shows he is a loving, concerned parent. That, in part, makes him a good parent. Bad parents don't care. Good parents worry. I'm sure he will think up a million things that he could have done different. But just like his stepson, we are all learning. Experience is a teacher where she gives you the test first and the lesson afterwards. As long as your husband uses this experience to learn, he not only shows he is trying to be a good parent now, but also that he wants to be even better in the future.
Finally, your husband is learning a hard lesson here. That is that sometimes we can see those we love making mistakes that we can easily see. We just want to drop our brain into their heads so they can just see what we see. And it is hard to watch loved ones screw up. It's real hard. But the lesson we have to learn is that sometimes people need to make their own mistakes. And it never gets any easier watching it happen again.
As long as your husband is always there for his son and is the best parent he can be, your stepson will see that. Maybe not in the near future, but he will remember later in life who was there for him, who was trying to help him. And hopefully that will turn your husband into a trusted friend of your stepson.