Is it appropriate to show concern for a professor's well-being?
He either has personal issues, or he doesn't focus on teaching.

If he is having personal problems, it is wrong to approach him like a friend and expect him to give details. You are the student and he is the professor, and his personal issues are private and he will share them with someone close if he wants to. As an adult, he probably knows how the process works.

Either way, the only thing that matters is whether he is doing his job properly. If you think he is not doing so, then you should give constructive criticizm, such as when you are in class and you don't understand, you can say that for the last classes you are having difficulties understanding and putting it all together and suggest a method so that you can benefit better from his course. Or, you can send an e-mail about how certain things can be improved without sounding very negative.