Translation:Mi hijo se siente cansado hoy.
The verb for "to feel (an emotion or a condition)" is "sentirse." It's a reflexive verb (you can tell by the "se" on the end of the infinitive), and each form needs its corresponding reflexive pronoun.
Yo me siento feliz.
Tú te sientes contenta.
Mi hijo se siente cansado.
Nosotros nos sentimos tristes.
Ellos se sienten enojados.
Then that means one should say "yo me llamo ____"? Of course the yo ain't needed here
And without the se he is physically feeling (sensing) some object---like the air, for example.
Which one goes smoother here, está or se siente? Both seem to be accepted.
If the verb is in the infinitve, it will end in 'se'.
If the verb is already conjugated, there will be an object pronoun immediately before the verb, and the verb will match in person and number. (In other words, 'me siento, te sientes, se siente, nos sentimos, vos sentís, se sienten').
If the direct object does not match up with the subject, then the verb is not reflexive, or not being used reflexively. Examples include "Te amo" and "Me gusta".
What kind of verbs are reflexive? Those that are transitive (in other words, they require a direct object) but the direct object is the same as the subject.
More specifically, verbs of self-care (lavarse, limpiarse, bañarse, ducharse, afeitarse, cepillarse) and verbs of self-action (ponerse, quitarse, sentirse) are almost always reflexive.
In English, we have the choice of saying "I bathed, I shaved, I got dressed" or "I bathed myself, I shaved myself, I dressed myself". In Spanish, you can only correctly say it the second way.
So how does 'sentir/sentirse' fit into this?
The former is transitive, used when you're feeling something or someone else. ("Siento tu dolor", I feel your pain.)
The latter is reflexive; basically, you're feeling your own situation. ("Me siento bien, pero él se siente enfermo", I feel fine, but he feels ill.)