Problem syncing iPhone to iTunes

I'm trying to help my husband sync an iPhone to iTunes on an old Mac.

After upgrading his phone to iOS 12 I got the message that the iTunes version was to old to sync. I upgraded his MacBook from Yosemite to Sierra which would give me an approved version of iTunes.

After the upgrade I have the following symptoms:

When I attach his iPhone it does not show up in iTunes, there are no error messages and nothing seems to happen at all. I have tried using different cables.

When I connect his phone to my newer MacBook running Mohave I get the trust this device question on both phone and computer so this is working fine.

I can sync his old iPad that is running iOS10.

I tried resetting Location and Privacy settings on his phone and then I got the "trust this computer" question on the phone but not the corresponding question on the computer.

I also tried to connect my iPhone running iOS12 using the same cable to his computer and got both security questions so versions are indeed compatible.

My conclusion is that I somehow need to reset the trusting of the device on the Mac?

Of course I would prefer not to factory reset my computer. I am looking to replace it but just wanted it to hang in there until this is done. Any suggestions?