I have a hard time believing that he's always sincere; it's seemed to me in the past that while he does seem to believe the things he says, the actual truth of them is not always there. Has anyone else experienced feeling this way? I feel bad about not giving him the benefit of the doubt and believing that he actually wants to and will improve, but I feel he's been down this road so many times before... .
It's really hard to trust someone who has let you down consistently. I'd be skeptical too, but as he's writing those words, that might really be how he feels... . and too much for him to live up to. He might be saying what he thinks that your parents want to hear--that's very possible. He might also be writing it down so it "sticks" and he's more likely to follow through.
Do the letters change how your relationship with him will be once he's out of prison?