Yes, as long as he is connected to the webinar, he is marked as attended. The Attendance Report will show the entire time from when he connected to the time he left the webinar.
If the GoTo app is in the background, his Interest Rating will be low.
Here is some additional information regarding Attendee data:
https://support.goto.com/webinar/help/how-is-the-attendee-interest-rating-calculated-g2w090036