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How many hours do you work during the week in total? How many hours is he adding per week?
Then, yes, you are entitled to overtime wages under normal circumstances (we don't know the full details of your occupation, you'd have to add some more info for a more specific answer). Generally any hours worked over the standard 40 in one week is considered overtime, and must be paid appropriately.
Note that it's perfectly legal for the employer to do this, so long as he complies with all relevant laws like the overtime statute.