"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:
Is there a difference in spanish between feeling tired and being tired. My son is tired was marked wrong.
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(I agree, a lot of work. It would be really nice if it could simply be done using the app!)
'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).