Looking for a watch-only plan with an option for international roaming

Hey Reddit, my teen son has an apple watch (with mobile data, not just wifi) currently on Verizon. I'd like to switch to something less... Verizony lol.

We're also going to be in Japan for 2 weeks soon so something with a reasonable intl roaming option would be stellar. Obviously it's just a watch so a shitload of data isn't really a concern but it would be great if it actually worked most of the time. Also being able to sign up without a phone on the plan is a must since parent phones are on a T-Mobile business plan.

Thanks for any advice!