Give us something to work with. Did you verify that the oil was at the full mark after the last change? The normal capacity of the 103 engine with a filter change is 6.25 quarts. Check your RO to see how many quarts you were billed for.
The mechanic "seems" to think there is an oil leak at the head(?). Either there is or isn't, and it should be easy to visually verify.
How many miles did you drive in those three weeks? A few hundred? A few thousand?
If the car has not had a noticeable oil appetite in the past, the guy probably just didn't put enough oil in it at the change.