Apple Watch range

My son has epilepsy and I am looking for a seizure monitoring device. He is 13 and I don't really want to purchase an iphone for him just yet. There are several apps that I am looking into that detect more than just falls. Can I use the apple watch without having a phone connected to it? I would need to be able to have the app transmit to me and my husband when his heart rate spikes and he does not communicate that he is ok. He sleeps upstairs and we sleep downstairs. Does the apple watch have to be pair with a phone?