Why is it spelled "התפוז" but it's translate into "His orange" when I thought the Hé/ה was "the" when placed in front on a word
Because of Shelo. If you have HaA ShelB, then A belongs to B. You have to ignore the individual meaning of Ha and Shel.
Good point that ה is the definite article "the," but idiomatically in Hebrew when they add a possessive pronoun (his, her, their), which is שׁל then they put in the definite article also. However, someone responded that sometimes the definite article is left out and it seems to be OK. So both are ok apparently.