No message alert for one contact

Hi there. I have an iPhone 8 with most recent update. I am getting message alerts for every contact in my phone book except for one. The do not disturb is disabled, when I scroll to the left on the message thread, it’s showimg alerts on and I have deleted and re-added my one contact. Still, everytime he messages me, I don’t know it until I open our message thread. Any ideas on how to fix this?

Many thanks!