I don't think you should be asking whether it is better to get into unnecessary debt by one method or another. It still has to be paid back, probably with interest. But as a child of the war years I'm sure you are aware of this.
Full marks to your son for opting to contribute to society rather than expect it to keep him, but I think you could advise he go the extra mile, as it were, and take a responsible attitude to his finances too. After all if he rewards himself week after week that debt is just going to pile up, and once used to treating himself it'll seem like some kind of punishment to have to forgo it later when the consequences become apparent.
Point out that if he stopped paying heavily for the times he's going over the overdraft limit, because he stayed in credit instead, he'd be more likely to save a bit with which he could treat himself without additional cost.