The Fighting Soul has to be blended with the warrior in order for it to become a part of the him. He must communicate with the power of the Fighting Soul for his own use. With this in mind, Su Mo had to think of how to blend his father's Spiritual Soul with this chosen Fighting Soul. For many other warriors, it was impossible to do this,since the Spiritual Souls of warriors simply could not blend with the Fighting Soul of others.
However, it might be different for Su Mo. He possessed the Devouring Fighting Soul and this might be of some help.
Su Hong's eyes were tightly shut and he looked excited. Su Mo concentrated his thoughts and the Devouring Fighting Soul instantly entered his father's mind. Su Hong's body instantly jerked, however he did not make any other movements. He completely trusted his son.