What is the ruling on the prayer of a congregant behind someone who does not guard against ritual impurity from his urine?

The purity of the worshipper, their clothes, and their place of prayer is a condition for the validity of the prayer. However, doubting someone's purity does not mean ruling them as impure, for Muslims are presumed to be pure until proven otherwise. Therefore, mere doubt does not permit you to abstain from congregational prayer.