Then, provided it was a "warrant without bail" (which it almost certainly would have been) he will be arrested and held until the next available court session when he will be taken there in custody. This could be the same day (if he is arrested in time to get him to court before their business ends), the next day or, if he is arrested after about midday on a Saturday, Monday morning.
When he appears the court will either sentence him as they would have done had he appeared on the due date or (more likely because it is doubtful his case papers will be available at that appearance) the matter will be adjourned. The court will decide whether or not to remand him in custody or grant him bail. The likelihood is, since he failed to appear, he will be remanded in custody.