Young Adult book with a Walther P38 pistol

I know this is vague and absolutely a long-shot. Here are all the details I can remember:

- YA novel about mental illness (depression)

- Teen boy is friends with his elderly male neighbour

- I believe their bond is based around old media, fairly sure they watch older films together

- A WWII Walther P38 pistol is featured quite prominently as a symbol/plot device

I will be honest -- this feels like a fever dream at this point. I found it in a neighbourhood lending library years ago, and read it in one sitting in a park while I was having a bad day. I passed it along to a friend, and forgot about it until yesterday. I've googled everything I can think of. You're my only hope.