In this case, the verb siente is being used reflexively. This means that the son is both preforming and receiving the action of feeling. Reflexive verbs are always immediately preceded by the appropriate reflexive pronoun, in this case "se". More information can be found here:
"Se" is the generic reflexive pronoun.
It is used all types Pronominal Verbs from Reflexives to Idiomatic: https://www.fluentu.com/blog/spanish/pronominal-verbs-spanish/
"Sé" almost always means "I know"
But rarely it is the "Tú" affirmative command of "Ser".
Once in a millennium it is the Imperative Mood for the "Tú" form of "Ser"--meaning "Be!"