I didn't want to ask you publicly, but where are you from in PA? I was born and raised near Pittsburgh.
You're doing right by your son. No matter what his bio dad says. By you explaining to him that his dad isn't around because he never learned how to be a good dad from his parents, you're setting your son up to know he needs to learn parenting skills and responsibility.
I know you're a great mom and you love him. I can tell by the way you want to protect him. It's exactly what I would do if faced with the same situation.
From someone who was basically raised without a dad until I was older, it did change me, but not in a negative way. Personally, I have been married to my awesome husband for 13 1/2 years (not bragging, I swear) and of course, we occasionally disagree, we have a great relationship. The most important thing is to teach him how to treat women. If he sees you're empowered, he will find a woman who is confident and empowered like you. It definitely sounds like you are. So, being his role model for success will set him up for success in the future.
Personally, my mom chose unsuitable partners when it was only up to her. When she met my step dad I was about 10. She asked me if I was ok with her dating and after I met him, if I liked him. When I approved of him, she was good with it. Now, my parents have been together for 20 years and married for 15.
By watching my mom go through some of the heartache she went through was difficult, I learned a lot just by observing. My childhood stuck with me and I remember so many disappointments of not having a father when I was small, but it also taught me to appreciate what I do have.
I definitely think you're doing right with your son. If his bio father is just going to drift in and out of your son's life, it's not him. As your son gets older, he will notice and realize that "Dad just doesn't have time for me, so I don't have time for him." It's not an easy lesson to learn, but valuable lessons usually come at a cost. It will make your son a better father in the long run, especially if you surround him with men who are also good role models. He will start to mimic their behaviour.
On a side note, my husband is so much like my father in law. It's seriously adorable. I swear, the older he gets the more he acts and responds like his dad. My father in law is a pretty awesome guy and I know he loves my mother in law. So, when I observe these little things, I realize how important it is to be around good people. I know my outlook on treatment of people has changed because of my husband. He was raised in a traditional family, but I really think the most important thing is to teach a child how to give and receive love. It sounds like you are doing exactly that and I know your son will be a great father and role model some day.