There are a number of reasons why he might be re-coursed. It basically means he's removed from his training course for some reason (and will most likely be offered another opportunity to attend the course unless he did something very bad).
If he is a reservist (part time soldier) he would likely get sent home. Probably put on a bus, train, plane, rare instances have a driver bring him back. They may keep him though and employ him in another job until the end of his contract.
If he is a full time soldier (regular force) he may be removed from his course and put in a platoon of other soldiers waiting until the next BMQ course starts.
There isn't much you can do at this point except wait for him to reach out and contact you with more details. If it was an emergency someone from his chain of command would have contacted you to speak with you.