Advice Requested

Here's my quandary (a very rare peak into the details of my dating life) - I spoke to a guy on the phone this past week, and based on our correspondence up to that point, and our phone call, I decided that I don't have any interest in continuing to talk to him. The reason that I don't want to continue is something that I feel he could work on, and if changed, would make him a much better marriage prospect and would allow women to see him in a much more positive light. (Just so you know, this has nothing to do with physical matters like losing weight or dressing better.) The question is, do I simply tell him that I don't think that we are for each other without specifying the reason? Or should I be specific and take the chance of possibly hurting him, and making him think I am critical of him, in the short term; but with the possibility that if he listens to what I have to say, then it really would be beneficial in the long term? Reader advice would be appreciated.