Late" mourning

I suggest a session on the question: How do you grieve long after the death of a loved one?

My husband lost his mom when he was 20 and is now 40. He's now 40, and thinks his depression is partly due to the absence of this necessary mourning. He says he'd put the subject aside for 20 years because he didn't know how to grieve. Today, time has erased the memories, the lack of a loved one and the regrets. So how can he free himself from this trauma?