to use his phone as a presence sensor, you have three options.
1) The official SmartThings mobile app
Get him his own smartthings account.
Invite his SmartThings account as a guest to your account
Open the SmartThings app on his phone, sign in as him, and then follow the following instructions:
https://support.smartthings.com/hc/en-us/articles/200292279-How-do-I-add-my-phone-as-a-device-
2) use life 360
A popular alternative that doesn’t require having the smart things mobile app on his phone is to use the free life 360 service. There’s an official integration with SmartThings for that.
https://support.smartthings.com/hc/en-us/articles/202334244-Connecting-Life360-with-SmartThings
3) use IFTTT presence to turn on a virtual presence sensor in SmartThings
This one is a little more complicated to set up, but some people like it for housemate situations because it doesn’t let you track the other person, it just indicates whether they are home or away. ( Life 360 is a service intended for families and so you can check on the other persons location throughout the day)
You can read about details for this and other options in the presence FAQ