I don't think he was doubtful about his firstborn son, I think he just wanted as many sons as possible to help run the family.
Exactly.
Anthony was 7 or 8 at this time and there's no reason whatsoever why Michael should be 'doubtful' of his abilities to one day run the 'Family'. As with many families in that time, while daughters were to be cherished and cared for and doted upon, they were little more than window dressing. It was SONs that counted, SONs that would inherit the Business and the Family... and Michael wanted SONs. As many as possible. Kay knew this, and she knew why. And that is why she couldn't bring herself to pop out any more for him.
