The son

After Kamala’s death, Siddhartha tries his best to provide for his son, but his son is a spoiled little brat. 
"the  young bird is used to another life, another kind of nest. He is not like you. He has not, like you, run away from wealth and the city of revulsion and a sense of excess. He had to leave all that behind against his will." (Hesse 124).
Siddhartha tries to convince him that all luxury and materialistic things in life have little meaning, but fails. He tries as hard as he can to make his son happy and show him how to live a good simple life, but it just ends up making his son more angry and unpleasant.  His son's anger builds up by day and one night finally erupts and he yells at Siddhartha. "I'm not a servant. Yes it's true you do not beat me-- because you do not dare! What you do is constantly try to demean me and punish me with your piety and kindness. You want me to become like yo, just as pious, just as gentle, just as wise! but hear me: to your sorrow, I would rather be a bandit and a murderer and go to hell than be like you! I hate you! You are not my father, even if you were my mother's lover ten times over!." (Hesse 130).  His son runs away the the next and Siddhartha over looks the city rand remembers all the great times he spent there with Kamla, then he realizes he must let his son go, and his lifestyle isn't fit for his son. Siddhartha didn't realize he was trying to make his son in his own image, but his son did and that's why he resented him and ran away.

OM

"Fear struck deep into Siddhartha  SO this was the way things stood with him! He was so lost, so confused and forsaken by all wisdom that he had had as a child to d=find peace, had grown so large he had sought it in the dissolution of his body." (Hesse 93).

Siddhartha thought he had experienced all that life had to offer, that he obtained all the wisdom there is, he had experienced all the happy things and love, as well as all the pain and suffering; and there isn't anything else for him for that he feel depressed and tries to drown himself in the river. But as he starts to drown, fear strikes him and he remembers OM, the very holy words that drove and inspired him to begin his journey away from home. He sits by the river bank, repeating OM, and he has awakened , once again.