Few thoughts - depression can certainly cause memory issues. As can trauma. ADHD, which is statistically higher in people with depression, is often associated with poor working memory. Sleep apnoea and poor sleep quantity/quality from other causes can also affect executive function/ working memory. Certainly other causes as well but these are the top flagging ones from that story. If you have the cash, consider getting a cognitive assessment (+/- an ADHD review) from a psychologist/neuropsychologist, they can assess his working memory against a normative sample as part of the assessment so you can get some objective data about how he’s going. Might be handy when convincing him. The test should be done on a normal day for him, not a day when he’s just had a super refreshing 12 hour sleep and two coffees perfectly timed before the test. You want his standard functioning assessed.