Probably not. When he was hired he would have filled out a form for the use of payroll in determining his withholding. If that form indicated an expectation that he would pay no taxes at the end of the day, then his only deductions would have been EI (Employment Insurance) and CPP (Canada Pension Plan). Well, I guess you could call those taxes, but they are separate line items from income tax withholding.
As I remember, he paid about $420 in tax. He gets it all back.
If it is possible under the Canadian tax system for both you and his mother to claim a tax deduction for him, and if she has in fact been claiming one then, yes, she will notice exactly the same effect that you do.
On the other hand, if you are the only person who has been claiming a tax deduction for him, then you are the only person who is going to be affected by the adjustment of the tax deduction to reflect his earnings.
It’s not of immediate relevance, but he’ll also have picked up 18% of his earned income (in this case about $2000) in RRSP contribution space, which will certainly come in handy down the road.