Activation Lock is designed to prevent anyone else from using your 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 can remove the lock. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. Dispose of the watch properly. In the future, see the 'Before you buy a watch from a third-party' section in:
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
If your ex-boyfriend the ORIGINAL owner (and you are still cordial with him), then he can see the 'Start a support request' section in:
How to remove Activation Lock - Apple Support