A dad isn't defined as the man who makes the child, but rather the man who extends his hands and time to help with the child's raising and his heart to love the child through anything! BLOOD doesn't always make a man a dad, being a DAD comes from the heart.
Ethan's father is not part of his life at all, due to leaving his mother after being abusive in the relationship. This quote contributes to his feeling of emptiness, like a piece missing literally and mentally. A father is a figure that children abide by and look up to, especially a son. His mother is affected as of he, contributing to his depression and anxiety. His dad has no input in his life so it leaves unanswered questions and thought provokes him into blaming himself.