When you prepare your son's tax return, it will generate and amount on line 23600 of his T1. If you haven't prepared his return yet, you can put in an estimate for now, and then put in the correct amount before you file.

I don't understand the question for my dependent 

Net Income ( T1, line 23600

He is 17 yrs old...

 Still in school 

Is he considered a student 

@Bruce Cook Line 23600 is your total calculated net income. This line is not shown on your T4 but it includes your Employment income  from box 14 seen on line 10100 in the Detailed Tax Summary.