Through this the author simply wants to indicate to the fact that ( a man may be the father of a son's flesh ) a man may give birth to his son may be responsible for his physical features his looks, but that definitely didnt mean that he was ( also the father of his spirit ) also responsible for his inner soul In our daily life too we see a lot of such cases somebody may resemble his/her parents in looks but be completely the copy of their uncle or aunts in their day to day behaviour. 

This means that genetically the blood of the father runs in the son's body, but there may be some variations in the genes, of the son which gives him a different psychological and emotional mindset which might be starkly different from that of the father.

Explanation:

The given lines mean that being a biological father does not necessarily make someone a spiritual father as well. These lines emphasize the importance of the emotional, psychological, and spiritual connection between a father and his son.

Biological Father vs Spiritual Father:

The concept of a biological father is based on genetics, where the father's DNA is passed on to his offspring. On the other hand, a spiritual father is someone who plays a significant role in shaping a child's character, values, and beliefs. A spiritual father can be a biological father, but it can also be someone else who has a strong influence on the child's life, such as a mentor, teacher, or coach.

The Importance of a Spiritual Father:

Having a spiritual father is crucial for a child's emotional and psychological development. A spiritual father provides love, guidance, and support, which helps the child to build a strong sense of identity and self-worth. A spiritual father also serves as a role model, and the child learns important life skills from him, such as resilience, perseverance, and compassion.

Conclusion:

In conclusion, the lines "a man could be the father of his son's flesh, but that did not mean that he was also the father of his spirit" highlight the importance of the spiritual connection between a father and his son. Being a biological father is not enough; a father must also be a spiritual father to provide the emotional and psychological support that is necessary for a child's healthy development.