Responses
SPOILER ALERT
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The daddy is the Crown Prince (Mettel/Maetel). He is actually quite aware and suspicious of the symptoms Louis showed during the interactions. If you have read all of the available chapters, for better understanding, I suggest you to pay detailed attention to Maetel expression everytime he is with Louis. Thus, you will notice that he is aware.
Regarding your question, when will the baby daddy, in here it means Maetel, he will certain that Louis is pregnant around chap. 35.

Can someone tell me when the baby daddy finds out he's pregnant and who the baby daddy is? Is it the prince? The one with the blonde hair? ( ̄∇ ̄")