What is the ruling on the liquid that frequently exits the penis, and is it obligatory to wash the penis and testicles and change the underwear, or is it sufficient to wash only the head of the penis?

It appears that the liquid mentioned by the questioner is not semen. Whatever its type—whether madhy (pre-ejaculatory fluid), wady (a thick white fluid that may be discharged after urination), or something else that occurs—it is impure (najis) and nullifies wudu (ablution). One must perform wudu after it, along with washing the penis only, unless it is madhy, in which case the entire penis must be washed, according to the opinion supported by evidence. It is recommended to wash the testicles as well, according to the more sound opinion. As for the clothes it has touched, it is sufficient to sprinkle them with water.