Apple

You can use family setup to set it up for someone else with your iPhone if you have one

Yes you have to have

While you can set up as family without him having an iPhone, he will need his own phone for use of the health app as you described. Also if he’s a fall risk, consider the cellular model because it can call emergency without a cellular plan.

It is easy to find a cheap iPhone. He wouldn’t even need cell service to use it with the Watch, it could be used as an at home Wi-Fi device only. Refurbished would be fine if you are smart about checking to make sure it isn’t locked to someone’s account. Places like tracfone often have new but older models and if you hit it right there are sales stacked with coupons.