The fact that he volunteered the information about never wanting to return to his ex, even if she were single, would indicate that whatever happened between them has had a lasting effect on him.
Even if he told you that he is completely over her, the fact that he brought her up without any prompting from you may be a sign that he wasn’t being entirely truthful to you.
If he was completely over his ex, she really shouldn’t even be part of his daily thought process any longer.
However, he brought her up in conversation which would indicate that she is still a part of his daily thought process.
As you stated, he may also be hesitant to get any further with you because of his 12 year old son.
Yes, he may worry about who he allows into his son’s life.
However, he may also prefer giving all of his love and attention to his son right now.
This is safe territory for him.
He knows his son and is able to shape how his son experiences the world and the lessons that he learns.
This is control.
If he were to allow a new romantic partner into his life, he would really have no real control in terms of how that relationship will go.
So much will depend on the cooperation and authenticity of that new partner.
Again, this is an area that he will have no control over.
Hence, it may be a lot easier for him to avoid taking things further with you by convincing himself that he can avoid a lot of hurt and maintain a lot more control if he just focuses on giving all of his love and attention to his son.