How should i address my physician's assistant?

Either Ms. or Mr.: They should have a name, and addressing them formally is proper, with mr. Or mrs. (name). They are not a doctor, so addressing them as such would be wrong. However, you can never go wrong just using formality to start with. People love hearing their names, and extended courtesy in general.

Mr or Mrs or Ms: Mr or ms or mrs is perfectly fine. Some like to be called by their first name.

Politely: It is reasonable in fact to call your PA by the name doctor even though they have a different degree. They are used to that, usually wear white coats and have stethoscopes hanging out of their pockets so they present themselves in this context as a doctor.