Hi S..
Although our situations are not exactly the same, I can definitely relate to your concerns.
I was never married to my almost 4 yr old son's father and we are no longer in a relationsip, but he does remain a part of my son's life.
I am now dating another man, and I have the SAME concerns that you have. I worry that somehow, he will let my son down. I worry that he and my son won't have a positive relationship. I worry that if we had another child, would my son somehow feel less a part of our family because he comes from a different father? Will the boyfriend ever REALLY accept the child and love him as his own? Is that even possible? Will we end up breaking up and cause my son to feel deserted by this man? Luckily, so far my guy is proving himself to be right for me and my son. But I continue to have lingering fears that creep in from time to time, but as we spend more and more time together, the fears are becoming less, and I am developing a trust in my guy...
Those are natural concerns - concerns any good mother feels when entering into a relationship that WILL affect their child. As you said, your son will ALWAYS be first in your life and you would do whatever is best for him.
My advice to you is the same as Lori's. This man has to PROVE to you and your son that he is WORTHY of both YOU and YOUR CHILD. And he will have to EARN your son's RESPECT and LOVE. It sounds like he is in the process of doing this already. :) Take things SLOW. Use your HEAD, but trust your HEART.