So, the problem is that you feel compelled to say something helpful to him, but don't feel you have anything to say.
The thing that stands out to me in your question is that you say you don't want to give him "general advice", or platitudes because they are "no use" to anyone. And you are absolutely right. That is a very wise observation for somebody who claims to have no life experience!
It sounds to me that you might be a "late starter" in life - that is you may not feel you experienced as many things in your youth as some other people - but that does not mean that you have learned nothing. Experience is amassed through a combination of both good and bad experiences and through the results of both good and bad decisions. You don't need to expound on your "broken childhood" but the fact that you understand and acknowledge that your past may have been different shows that you have progressed and learned at least some important things along the way. Basically, I think you are being too hard on yourself and you can probably do a better job of helping your nephew than you imagine. If you know or think that something about your past or upbringing was wrong then you must have an idea of what it should have been like, so advise based on that.
You don't state whether this request is for one single talk or if they expect you to "mentor" him over a period of time. In either case, agree to meet and talk to your nephew. Perhaps say:
I want to help you the best that I can. I may not have had all the same experiences and opportunities that you have but I have learned a few things and I can share those with you.
Don't just launch into a lecture though - ask him questions about himself. His likes, his goals etc. To help the way you are being expected to you need him to open up and trust you. The advice you give then does not have to be random - you can respond to the things he tells you.
Quality advice has usually been put to the test and can demonstrate an outcome. If you did something and it turned out good, say what you did and how it turned out for you. If something you did turned out to be a bad choice, or if you missed out on something then state why he should take full advantage of it if he has the chance you didn't.
If you are expected to speak with him on an ongoing basis then talk about the things he has done, whether he followed your advice or not, and how things turned out. Ask him how he might do things differently. If you don't feel you have the knowledge to guide him you can still act as a sounding board and help him to figure out his own way of doing things by examining his actions and the outcomes.