Why does my boyfriend have onset of depression after the birth of our 9 month old son.?

As much as a child: Is a blessing - this also brings major life changes. There are new, major responsibilities for the parents. The father may correctly perceive that attention he used to have from you is now directed at your child. Sleep deprivation can take it's toll. It may be helpful for your boyfriend to sort this out with a counselor or even a pastor.