Firstly, if he drinks half of the water, the amount of water he has left is 2 - (0.5*2) = 2 - 1 = 1L.
Milli is the SI prefix for 10^-3, or one-thousandth. This makes 1 millimetre one thousandth of a metre, and consequently one millilitre is one thousandth of a litre too. The converse is that there are 1000 millilitres in one litre.
The answer is then 1000ml. I hope this helps you :)