Apple Watch & iPhone 6 pairing issue

[DMC2]

My son has my old iPhone 6 and a new Apple Watch Series 3 that he got for his birthday. I can't get them to pair, just keeps saying the phone needs to update (but it is fully up-to-date). I'm guessing this is because the phone is now too old. I've read that the watch needs to be on the older system, but only Apple can do that.

Is there anyway round this? For example, can I put his sim in my phone and sync it with that, then put his sim back in his phone?!?

[geeks]

The 6 doesn't get iOS 13 so I don't think there is a way around this if the watch is already on the later version if WatchOS

[ecsrobin]

You need an iPhone 6s or later to support an Apple Watch 3.