The reasons I am leaving include not being assigned the duties I was hired for ("other duties as assigned" is the majority of my work), being underutilized, not being trained in relevant areas, and generally poor communication throughout the company. I have expressed these concerns to my boss in the past
These are symptoms of the real problem: The job is not a good fit for you.
Face it - you've attempted to create a better fit by bringing your boss's attention to the frictions in the job, and they haven't changed appreciably. You no longer see opportunities in the position to increase your skills and career in the desired trajectory. You know there is a position that will better meet your goals, and it's likely that there's a person who will fit this position very well.
It's not necessarily that the company is bad or wrong, or being mismanaged. It's not necessarily that it's a bad position. It may be that it is those things, but from your perspective you might not be able to objectively make that call.
So when you say, "I'm being asked to do work I didn't expect to be doing" or "I'm not being trained enough" or "People here are bad at communication" then it's just as likely that your boss will blame you for all these problems as himself or the company. Since you've talked to him about them before and nothing has changed, it's very likely that these things don't concern him. In his eyes they concern you.
So there's nothing you can gain by telling him, and little to nothing he will gain.
Worse, though, is that for the next decade or longer they will continue to appear on your resume as previous work experience. While it's possible that they actually never give more information than a confirmation that you worked there, some companies do give feedback - sometimes negative - when future employers call to verify your employment history.
By telling him all the bad things you blame the company for, he may see that as your failing, and that may be negative for you in your career.
Unless you have a personal stake in the future of the company, and you can be certain you are absolutely objective in your assessment, you should not go any further than "I have an opportunity to grow my career in a new direction. I've learned a lot here and will miss the people I worked with."
This distances both you and him from the idea that there's something wrong with you, the position, or the company. It also avoids comparing the old position and the new, avoiding questions about why you prefer the new one to the old. It is also a standard 'canned' answer that allows your boss to fill out his forms easily and simply, reducing his work in your leaving.