If a person performs salat asr and then after the time for the salat has become qadha (after maghrib) has realised that he forgot to perform wudhu, does he have to perform qadha of that salat? At the time of performing the salat he was sure that he was on wudhu and only realised several hours later (after maghrib) that he hadn’t performed wudhu.

If he doubts whether or not he had performed wudu he should ignore his doubts. However if he knows for sure that he had not performed it than he would need to perform it and Make up his prayer.