SIstani says: “Ruling 346. If some fluid is discharged by a man who is not sick and that fluid has one of the three characteristics mentioned in the previous ruling, but the man does not know whether or not it had any of the other characteristics, [he will not be junub and] in the event that he had wuḍūʾ before the discharge of the fluid, that wuḍūʾ remains valid. And if he did not have wuḍūʾ, he only needs to perform wuḍūʾ [when required], and performing ghusl is not obligatory on him.”
I understand that he is not considered junub, but since he is not sure if it is semen, does he treat the liquid as najis and must change his clothes?
The fluid is not considered semen in case which is mentioned in ruling 346, therefore, it is considered pure (tahir).