I'm sure you want to know why he ended the relationship, and you did everything to ensure the relationship is active. But, think about this: He 'dumped' you for another girl. I wouldn't suggest you meet up with him to find out why he ended the relationship. He might even not tell you the truth so as not to hurt you.
I know you'll be psychologically tortured to know why he broke up with you. You would want to know why and what you could do to get back your ex.
It is obvious before he ended the relationship, he was seeing the other girl (maybe for a few months or weeks).
It's better you don't know because your ex used an inappropriate method to inform you of his wanting to end the relationship.
About the child, let him know through text or call him. It's sad he wasn't there for you and the child (and he is right he has put you through a lot. You had to take care of the child on your own).
Find the appropriate means to let him know he has to forget about his son (until an appropriate time if you wish so).
Don't contact him for a few days. A week is appropriate. After one week of not contacting him, contact him through text or call him and let him know of your decision (you won't allow him to see his son. He needs to forget about him until when the child would be ready to know his father).