Why does a man's depression in the face of a major life change have to be related to childbirth at all? Why not just call it depression? It's really no different, NICE are clear on this. For the majority of women, there are additional physical and social challenges at this time that are NOT experienced by men. Breastfeeding, for example, is a major source of rumination and distress for women that men will NEVER EXPERIENCE.
I fully believe a man can experience depression triggered by becoming a father, having a child, change of relationship with his female partner, additional stresses and strains, worry about finances etc... but many of the most common themes of rumination and distress in PND are generally experienced either exclusively or far more frequently by women because of the differences in gender or sex role - ruminating about the birth experience, breastfeeding, body change, resentment about suddenly being expected to take full responsibility for housework, isolation on maternity leave, lack of support for the role of mother, being with a baby 100% of the time, 24-7. Having absolutely NO time away from the baby or young child is a very common experience for many, many women and complicates recovery for many women who get PND. There are very, very few men who would be in a situation where they had absolutely no time away from the needs of their young child across the 24 hours of the day.
In all my treatment (and I had a LOT) it was the woman-specific treatment I had a PND group that went the furthest towards making me feel normal again, because we had the opportunity to explore some of these themes which all of us, eight women in extraordinarily different circumstances, shared.
I just don't think it's appropriate to make male depression at this time of life about pregnancy or childbirth. PTSD after a traumatic childbirth is somewhat different, but again, there's no need to make the label about the birth - there's no implication that depression or PTSD are not serious or worthy of treatment by not calling them "post-natal".