Depending on the type of infection and the type of antibiotics I would expect him to have a noticeable improvement within 48 hours maximum. In severe cases this can sometimes be up to 72hours. If there is an improvement he must still continue using it until he has taken the full dose prescribed. He should also practice good ocular hygiene and not risk infecting his other eye if at all possible.
If, however, his symptoms are significantly worsening over the next 24 hours he should consider getting it re-examined to be sure he is being given adequate treatment.
Hope that helps, and hope he feels better soon.
H