Out of combat characters can use encounter powers once every 5 minutes. So yes, he needs to rest 5 minutes, then he can use them again, after which he needs to rest 5 minutes again if he wants to use them in the next encounter...
Except that some powers can be used twice per encounter, such as Healing Word, in which case such powers can be used twice every five minutes (use twice, take a short rest of at least five minutes to recover encounter powers, use twice again, take another short rest of five minutes, use twice again, etc.)