Long run, he should accept the will of God/nature/whatever-he-believes-in and love the daughter fully. Period.
That said, short term, it is OK to be disappointed. Emotions happen. He should let himself experience his emotions, then move forward. If he needs to mourn the image of all the things he wanted to do with a son, he can go ahead and mourn the image.
If he needs help doing that, he can try counseling.
If he can't accept what God/nature/whatever-he-believes-in is going to give him in the way of gendered children, he shouldn't have them.
Long run, gender is such a small thing. It tells you very little about what the child will really be like. We don't get to pick the personalities, abilities, looks or gender of our kids. We don't get to pick anything, really. Name of the game is accepting and loving what you get.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).