I feel like my fiancé doesn't like our son as much as our daughter and it irritates tf outa me how…

It sometimes take men a while to adjust to a new baby. He may just have more fun with your daughter cause she's a toddler and can interact more. Talk to him about it and give him time to warm up.

That's not the case at all I promise as your son gets older he'll be more involved. Guys just do not know what to do with babies that young