Just delete the app from your phone. Check the web app once or twice a week if you want. You'll probably realize there really isn't much to be gained from checking instagram everyday.
My phone's ROM has Facebook and Instagram baked in. Unless I trust some unknown person's custom ROM, I'm stuck with Facebook properties doing god knows what on my phone.
How do you measure that (so accurately)?
In "Screen Time" (IOS) you can see how long you spend on social networking apps
Apple screen time breaks down the time you use each app very well.
If he is using the Instagram app, and Apple screen time or similar android feature it can give you a report based on per app time.
Feature of the Instagram app. On your profile page -> hamburger menu -> the one with the clock. (I have all the texts in German. Maybe something about "activity" in the normal version?)