He should have received a letter from the employer (and normally he would have been told the same, prior to being sent home) that he has been suspended on full pay, pending an internal investigation into the circumstances. As part of the investigation, he should be invited to an investigatory interview (which is not the disciplinary interview and it happens beforehand) for the employer to hear his side of the thing that they are investigating.
3 weeks is not necessarily a long time - it is not unknown for public sector employers to have employees on 6 months paid suspension whilst they phaff about investigating. However he could contact the HR department for an update if he wishes.