No sweat Lara,
So scream at us anytime, as much and as often as you want: "You're bio dad is a jerk!" I think, although I'm not there yet, that one of the important jobs of a good parent is keeping healthy boundaries--our kids get to vent to us, act out and tell us whatever they want to. But we shouldn't do the same to our kids...I'm not saying that we should lie to or keep stuff from our kids (I found out adopted dad was not bio dad from a fellow classmate in 3rd grade...not how you want your kid to find something important out), but there's lots of stuff that we appropriately discuss with husbands and friends, not our children. Keeping those healthy boundaries is exactly what you are doing when you tell us instead of telling your son that his bio dad is a jerk.
It also sounds like you're having some healthy conversations with your son about unwed fatherhood, the responsibility of parenting, and what makes a good parent. My father, too, is a charmer. For the first few years that I knew him (from age 23-25), he came on like super dad being very involved in my life and very generous. He sent really thoughtful gifts and sometimes even Fed-Exed letters that he was too impatient to send via regular mail. At first I was seduced and felt really angry that I had been kept from this wonderful man, but as I got to know my siblings and see how he treated them, I began to see the bigger picture. He couldn't sustain it and now we're virtually estranged. I still value some of the special things that he did, but I also see his many flaws and have a lot more empathy for my mother than I did before. Someday, your son will probably see his father as a whole, not perfect person as well. As your son becomes a man, especially when he starts a family of his own, he'll probably realize his father's shortcomings without you having to tell him. Then, he'll appreciate your restraint.
Also, for another supportive anecdote. My uncle died when my two first cousins were very young--like two and three. My aunt remarried not too long after, and her new husband was a father in all the important ways to my cousins. During childhood and adolescence, they complained bitterly about their stepfather, but now that they are adults (early thirties), they really appreciate him. The younger just named her second son after him.