I bought an Apple Watch 1st gen and i need help.

You have encountered Activation Lock. See this and have the original owner take care of removing Activation Lock with the instructions here under Turn Off Activation Lock. About Activation Lock on your Apple Watch - Apple Support

If they no longer have access to the phone it is paired to, then try this.

If you can't access your iPhone or Apple Watch, or if your Apple Watch isn't paired with your iPhone and you need to turn off Activation Lock:

1. On a computer, go to iCloud.com and sign in with your Apple ID.
2. Go to Find My iPhone.
3. Select All Devices, then click your Apple Watch.
4. Click Erase Apple Watch. Select Next until the device is erased.
5. Click next to your Apple Watch.

I don't know exactly, but I'm guessing it has something to do with the activation process, and it has to check against the Apple servers, just like with Activation Lock on a phone. When you go to pair it, it has to check against the Activation servers to see if it is Activation Locked, and doing what I provided through iCloud will clear it at Apple's end.

You're welcome.

He doesn't need the phone. Sorry, see the section here for using the Watch. If you forgot your Apple Watch passcode - Apple Support He should be able to reset the device. If he cannot remember the passcode for the watch, then he can use this document to reset the device.