Put the Mg in the bottom of a 3-necked round bottomed flask, have a septum over one of the necks, a stopper in another and your gas syringe connected to the last.
Squirt in the sulfuric acid through the septum from a syringe, accounting for whatever volume of gas this will push into the gas syringe.
Allow everything to cool to room temperature after the reaction has subsided, then measure the volume.