This app was mentioned in 8 comments, with an average of 1.50 upvotes
https://play.google.com/store/apps/details?id=com.smsrobot.callrecorder
Here's the play store link. The name and icon in the store is different from what is downloaded on your phone for some reason but I've loved this. Doesn't work well with headphones or bluetooth but otherwise it's great 鈽猴笍
I am still able to record on my Pixel 2 XL with "Call recorder - Automatic" after upgrading to Pie. It seems to be no different than it was before. Edit - Added link to app. https://play.google.com/store/apps/details?id=com.smsrobot.callrecorder
I'm going to preface this by saying that I'm not a lawyer or even a legal student, so this is the most amateur of advice.
I would start recording phone calls, as far as I can tell both parties need to know that the call is being recorded for it to be legally used by 3rd parties, but there's a lot of great free apps for your phone that will record all incoming and outgoing calls automatically.
I have an Android phone and use CallX, it's completely free and very good.
Every time he calls up let him know it's being recorded, if he refuses to talk while being recorded then refuse to talk to him. It might make him hold his tongue, or if not, then you'll have something solid and admissible in court.
https://play.google.com/store/apps/details?id=com.smsrobot.callrecorder
works on my S9+ and S10+. YMMV
Before I had my Pixel I used this. Worked great on my Samsung S7. https://play.google.com/store/apps/details?id=com.smsrobot.callrecorder