My brother iPhone X can’t pair with Apple Watch series 10

That's correct. The Watch 10 requires an iPhone running iOS 18. The iPhone X is NOT compatible with iOS 18.

He will need a newer phone in order to use that watch.

As KiltedTim had mentioned, you’ll need to upgrade the iPhone for compatibility.

Once that is all done, then follow these steps: ⇣

First, make sure that both devices are up-to-date:

• iPhone Software Update.
• Watch Software Update.

Then make sure both devices are in close proximity with the following settings:

• AirPlane turned off
• WiFi turned on
• Bluetooth turned on

Click ▶︎ Connect/Pair your Watch for detailed instructions.

Need more help?

• Call 1-800-APLCARE.
• Contact Support.
• Apple Watch Support.
• Apple Watch User Guide.

Good luck! 👋🏼😉