Praise be to Allah.
If that happened whilst he was sleeping, then his fast is valid, whether it was maniy or madhiy, because emission of maniy whilst sleeping does not invalidate the fast, as has previously been explained in the answer to question no. 14014.
But if that happened during the day, then it is possible to differentiate between maniy and madhiy by knowing the characteristics of each of them. Maniy is emitted forcefully and gushes, and is thick, and has a smell like dough. As for madhiy, it is sticky and thin, and has no smell. Please see the answer to question no. 166106.
If there is uncertainty as to whether it is maniy or madhiy, then it is to be deemed madhiy and the fastest to be deemed valid.
The scholars of the Standing Committee were asked about someone who emitted something after playing with his wife whilst he was fasting. They said: If the situation is as described, then you do not have to make up the fast or offer expiation, because matters are assumed to remain as they originally were, unless it is proven that that wetness was maniy, in which case you must do ghusl and make up that day’s fast, but no expiation is required.
End quote from Fataawa al-Lajnah ad-Daa’imah – vol. 1 (10/273)
And Allah knows best.