The Vivitrol shot will block the effect of opiates when opiates are used in moderation. Some will continue to use at higher dosages to feel the effect and when they do so, they substantially increase the chance o for OD. It's far more common for an addict to switch to non- opiate based drugs, typically cocaine or its derivitives.
The real value of Vivitrol shot is that unlike Suboxone, it cannot be traded for the drug of choice.
If he works the program, as established, it will work. His recovery or not, is an inside job.