"Mi hijo se siente cansado hoy."
Translation:My son feels tired today.
You are mixing two different things, this sentence is indicative not subjunctive.
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:
'Sentirse' is a reflexive verb. That means you have to use it with the corresponding reflexive pronoun. 'Se siente' is correct and means 'he feels', but 'te siente' is wrong and should be 'te sientes' (you feel).
Wo whats the difference between me siento cansado and estoy cansado. I feel tired and i am tired. Surely since tired is a feeling itself, "i feel" is redundant