Learn Portuguese (Portugal): Na minha opinião, ele tem razão.

In European Portuguese, it is standard to put a definite article before a possessive pronoun. So, you need a before minha. When the preposition em (in) meets the article a (the), they contract to form na. Literally, you are saying "In the my opinion".

Why does the sentence use tem (has) instead of a verb for "is"?

To say someone "is right" about something, Portuguese uses the idiom ter razão (literally, "to have reason"). If you try to translate English directly and say ele é direito or ele está direito, it would mean he is straight or upright, not that his opinion is correct.

Can I use estar certo instead of ter razão?

Yes, you can say ele está certo (he is correct), and it means roughly the same thing. However, ter razão is the most natural and frequently used expression when agreeing with someone's argument or viewpoint.

Could I drop the pronoun and just say Na minha opinião, tem razão?