Apple Watch 3 for a Child?

For a series 3 he'll need to have his own iPhone as well. To set up an Apple Watch for a family member without them having an iPhone it needs to be a series 4 or newer and it has to have cellular capabilities. You cannot use the GPS only Apple Watch.

Yes,if he doesn't have his own iphone then read here;

Set up an Apple Watch for a family member – Apple Support

I apologise,anypats is right and you do need an iphone,I must be half asleep.