Just where do b parents fit in???

My son recently met his b father. He (the b father) and his wife (not b mother) and two children live nearby. This man's children are much younger than our son, and, although he seems to be a good dad to them in a practical manner, he seems rather immature in his relationship with our son--almost as though he is another one of "the boys." Our son's relationship with his b father is difficult for us to understand. On the one hand, he makes comments like, "I've done fine without him for 16 years, and I'd do fine without him for another 16 years." There was an incident where he borrowed something from our son and took a long time to return it, and our son made a comment that he was going to have to "beat the c--p out of him" (our son tends to get a bit "mouthy" when he's angry) if it wasn't returned soon. Our son also made a comment that, had he been in his b father's shoes, he would never have run away and deserted a girl he got pregnant (which his b father did), no matter how scared he was (I have no doubt that this is true). I feel that he has very little respect for his b father, yet, he seems to enjoy his b father's company and visits at least every couple of days to, as he puts it, "hang out." We're okay with the relationship for the most part, but I'm a bit concerned about how this relationship might impact on our son. He doesn't need a father (and, in fact, there doesn't seem to be any father/son type interaction going on between the two), and he has his peer group of friends. So, just where does this man fit into our son's life if he's not a parent and he's not a buddy?