I am around age 50, from a divorced family with brothers, and parents who dated a lot, have studied a lot of psychology and witnessed many similar scenarios. Based on my experience, your situation with your son is, unfortunately, normal. Boys have a terrible time turning into well-adjusted men without the father figure, which you are figuring out. If you listened to Dr. Laura, for example, you would hear calls like this every day.
The only suggestion that I have about proper role-modeling is to get the young man involved with a church teen group. Men who are well aware of these issues generally run these groups, they provide positive role-modeling, and they have in-depth discussions which could help provide the guidance that he needs. (Some of these groups are run by women -- you want the ones with the men in charge for your son.) Church teen groups are full of all kinds of kids from the community, including kids who have never gone to church.
People hate it when I say this, because they don't want it to be true, but GENERALLY SPEAKING (I didn't say "all the time", right?) kids hate it when their parents date and there's very little chance that a teenage boy will accept a new step-father as a role model. (I'm going to get hate mail now. Whatever.) Most of these kids will not tell the parents that they don't like the new girlfriend/boyfriend, because they don't want the parent to be mad at them.