The mere sensing does not invalidate the wudhoo; nor is it permissible for him to depart from the obligatory prayer due to merely doubting. For it has been established from the Prophet (Sallallaahu ‘alaihi wa sallam) that he was asked about a man who discovers something whilst in prayer so he said:
((He should not depart until he hears a sound or finds a smell))*
As for if he is sure of the discharge of urine at the tip of the penis then his wudhoo has become invalidated – and upon him is to make istinjaa .
Except if he has the incontinence of urine in which case his prayer is not void due to that - for as long as he does as he has been commanded. And Allaah Knows best.
*Reported by Al-Bukhaaree in the book of Wudhoo number 137 and Muslim in the book of Menstruation number 361 and Aboo Dawood in the book of purification number 176.