SmartThings doesn’t really call them scenes, as far as I know. Which Smartapp are you using to create your rule tied to your son’s presence sensor? Can you restrict the hours that the rule is active so that it only changes the temperature if he’s home/away before 6pm?
You could always use Rule Machine.
Create a rule that uses the presence sensor as an input. If he’s present (rule is true) set thermostat to 75. If away (rule is false) 78 degrees. That can definitely be restricted to certain hours.