It could mean he is using again or has some unresolved issues that could require more therapy.

Yes. The lay term is "Dry-Drunk" a condition of being not using but angry and irritable. He should discuss this with his treatment provider who may assess him for depression or some other condition, or he may have relapsed, or is more likely to relapse. People in recovery may take some time to learn to feel normal with their brains off drugs, but long term, he should feel content and a better person from working on his recovery. Support him in whatever way is healthcare provider suggests.

It could be related to emotional withdrawal related to their addiction, or he may have dual disorder (depression and drug addiction), therefore he should have a follow-up with a psychiatrist and be medicated for depression plus drug counseling.