Multiple watches can be paired to a single iPhone, but not the other way around. It must be unpaired from the lost iPhone. Here are two ways to unpair your watch if you no longer have the paired iPhone:
- See the 'How to erase your Apple Watch if you don't have your iPhone' section of:
Unpair and erase your Apple Watch - Apple Support
- With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.
Both methods require knowing the Apple ID & password.
Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.
About Activation Lock on your Apple Watch - Apple Support
Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner (your dad) can remove the lock by using their Apple ID & password. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. Your dad can try:
If you forgot your Apple ID password - Apple Support
If he is the ORIGINAL owner, then see the 'Start a support request' section in:
How to remove Activation Lock - Apple Support