You have one "Lucky" Son who gets all the kewl toys 😀
Yes, your son is good to go. I was just not sure if the iPhone 12 mentioned in the first post was his, or just that one existed in the family.
I use a GPS Only Series 4 Apple Watch with my iPhone 12 Pro. Since my iPhone is basically glued my hip, I get all the Apple Watch benefits, without paying extra for a Cellular Apple Watch, and not having to pay an extra $10 a month for cellular service. I also tend to use low cost pay-as-you-go cellular service, that does not support the Apple Watch. So your son and I are basically in the same boat.
Then again, my Wife does not want her iPhone surgically attached to her body, so she has a Series 6 Cellular model (Christmas gift).