I am going to be the voice of dissent here. Your boyfriend spent the first 6 years of his adult life in prison. That most likely could not be because he "borrowed" a car or something like that; for an 18-year-old to receive that kind of sentence, he must have been convicted of something serious. Plus, the experience of being in prison for 6 years would in and of itself (even if I thought the inmate were innocent) make me reticent about a romantic relationship. I'm glad he has cleaned up his act, but I understand why your family thinks you deserve better.
You might be surprised at what could get someone a 6 year sentence.
I firmly believe that the justice system can and often does work. I also firmly believe that it fails regularly for many reasons. The most important is that the system simply cannot try everyone, there isn't the time and funding for it, so many people are pushed into pleading out. I have known people who got off on technicalities, and people who plead out even though I knew they were innocent (one had a whole fire station as witnesses and was still told it would be better to plead out because of the judge he got).
OP, you are choosing to explore a path right now. And I think it's wonderful that you can see that some people do stupid things well beyond the normal stupid things most people do in their late teens/twenties. On the other hand, it pays to watch out for yourself and not start down this path with out a bit of research.
I encourage you to ask him for a copy of his record, and then I encourage you to double check it (run a background check, not just a google check). Think about what it will mean for both of you. For him, it will be harder to find work, or even impossible in this economy. Will you be willing to support him? How will you feel about that, how will he? Are there other consequences?
How was he effected by time in prison? It can be a rough place, it can be an eye-opener, and it can be a place to turn one's life around all at the same time. If he had a rough time, are there long term issues with that?
Are you comfortable with his openness about it? There is a very strong stigma associated with jail time, especially for "blue collar" crimes. How will you handle your family? What will you tell your kids? How will you handle it when your daughter's best friend is no longer allowed to come over because the other parents don't trust your husband?
This is a major obstacle in his life. While it sounds like you are just dating
now, this is a good time to think about whether these things will bother you down the road. It may be easier to go back to being friends now if you decide you can't handle or agree on how to approach the issues that will come up.
I don't want to discourage you. (I don't necessarily want to encourage you either) I think you need to think long and hard though. It's not an easy road to walk.