I cannot know his particulars, but I can hazard a guess.
A relationship in the form of a spouse, children
even a pet brings with it
- a sense of purpose,
- a sense of belonging &
- an affirmation of being valued by another.
Even introverts value those.
Those same relationships, particularly the first two, bring with them a certain amount of unpredictability; an unknown quantity.
Autistics thrive on being able to order their lives into efficient patterns.
Other people (autistic or not) do not always abide by those patterns. (That is the hidden cost of maintaining such relationships.)
He may be coming to the realization that his life is about to become more complicated in ways that he cannot make a priori plans for, because the baby is still a stranger. Wait-and-see plans are a posteriority plans. They are fraught with uncertainty.
We have to learn to wing it!
We can still plan for the needs that we see, but must accept that there will always be unknowns.