How to amplify the output of an on NMOS, connected to zero source voltage?
The simplest way to amplify digital signals is adding inverters. If an inverted output is not allowed, you can add 2.

You have some control over the switching threshold by playing with the w/l ratios. Make sure your NMOS has more beef than the PMOS to have a low switching threshold. 

Downsides are possibly an increased area and/or propagation delay.

Alternatively, you can also move to a different XOR circuit that doesn't need a switch to work.

If you have both inputs and their inverted signals, you can also use



You can probably find more.