I’ve found that nothing motivates a stakeholder quite like watching a user or two try to do something (seemingly simple) on the companies app or website, and fall flat on their face.
I’d create a scenario for some users to go through (signup flow, and a few tasks post signup is usually a good setup), then pay for 5 users to go through the flow on usertesting.com. Select the two most painful videos out of the bunch, and then schedule a 30 min meeting with your boss. Sit with your boss and watch both of the videos all of the way through.
My point is, rather than point to external articles, drive it home where it hurts the most (your own user experience).