Sorry I haven’t read the novel either (my friend told me these spoilers) so im not sure how he found out that the child isnt his but the reason they thought navier was infertile in the first place is bc when sovieshu and navier were younger they both ate a drug-laced cookie together. The drug is used for miscarriage and one of the side effects of the drug is infertility, sovieshu didnt know the cookie was drugged while they ate it but he did find out later and he didnt tell anyone. Until trashta came there, sovieshu also didnt know if one of them OR both were infertile but after he found out trashta is pregnant, he just assumed that navier was the only infertile one.
Spoiler
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Sovieshit is the infertile one, not navier, and the child is from trashta’s previous lover so it’s not sovieshit’s child. Eventually he will find out, trashta will commit suicide and navier will live happily with king heinrey and their children while sovieshit will live in regret and wants navier back but nope he’ll live alone without her