If you want set up the watch and give it him to use as just a watch and activity tracker you will need to turn of notifications on the phone and all the mirror to watch settings. But it will not stop him from opening the apps on the watch and getting data from the phone, mail, messages etc.
You are better off giving him your watch and picking up a used 5 or 5c for him to use, or better, get yourself a new 6 or 6 plus and give him the whole package phone and watch