A warm welcome to you Wife_seeking_support
First, congratulations on the recent birth of your youngest. From one mum to another, it remains important you have some support around you, as well as a solid management plan with all that's going on.
Whether it's the person who's experiencing mental health challenges or their partner, it's definitely important to have a management plan. It's good that your husband is already seeing a mental health professional. I strongly suggest he mentions to the psychologist that the medication doesn't seem to be working. This way, the medication can be tweaked or a new one can be tried.
Depression is definitely complex but to simplify, it reflects mental processing and chemistry. The chemistry aspect can be incredibly influential in our way of thinking. For example, if healthy levels of oxytocin (the bonding or love hormone) aren't present in the mum of a newborn, she will not be able to bond lovingly with her child. If serotonin and dopamine are an issue (relating to happiness and a sense of reward), she will not feel the natural joy of life or a sense of reward in what she achieves. So, the thought processing involved in motherhood can be hijacked by chemistry resulting in postnatal depression. Seeing most of us don't see ourselves as walking miracles of highly complex interactive chemistry, the thoughts 'I am a bad mum' or 'I am hopeless' painfully play out. Until the chemistry changes (one way or another), it will prove almost impossible to alter this mother's perception and beliefs regarding her personal sense of identity.
The chemistry and thought processing your husband is experiencing sounds incredibly intense, therefor he does need to speak to someone who can help him get through this. As one of his supporters, it would pay to do a bit of research on depression and how it works. Coming from a holistic angle, I believe there are actually 3 aspects to it, as opposed to just mental and physical/chemical. Any quantum physicist will tell us that all life vibrates on a particular level or frequency of energy. From a spiritual angle, this belief is the same. If the energy of our mind and body are not 'vibing' with the higher energies of life outside of us, this can help explain that intense feeling of disconnection from life that can come with depression. Doing some research on the mental, physical and spiritual aspects may prove to be empowering for you in the way you can help your husband manage.
Take care
