Messages on Mac: Old addresses appear in autofill

There has been an extensive string of posts on this subject, but they are now quite old. I am on macOS 10.13.4 on my computer and on iOS 11.3 on my iPhone.

My son has two American mobile numbers and a British one. He is now living in Britain, as I am, too. When I type his name into the address field in Messages on my Mac, all three appear. What is much worse, if I reply to one of his texts sent from his British mobile number, my reply goes to one of his American numbers and then back to Britain, which costs him quite a lot of money.

I have removed his American numbers from Contacts in both my devices and checked that they have gone from iCloud also; and I have deleted all past messages addressed to him from both devices. I have also deleted them from past emails in Mail/Window'Previous. recipients.

I have, further, tried all the many suggested procedures in the earlier string of posts on this subject, in Mail as well as Messages and others. But these all related to earlier operating systems. Can anyone please help me to remove the unwanted American addresses in my newer operating systems, High Sierra and iOS 11.3?