In most of his interviews it actually looks like he is a Mclaren employee.
Here's a recent Chinese GP preview quote from NH;
Mclaren may be Mercedes' biggest client but shouldn't he be a little more impartial ?Norbert Haug, Mercedes-Benz Motorsport Vice-President
“Last year, Lewis drove an excellent race on this challenging circuit and won. For this year’s Chinese Grand Prix, it has to be our target to start further up the grid than was the case in the first two grands prix. Those two races showed that points-finishes or even podiums were possible, even when the basic speed was not good enough. The team has developed further technical and aerodynamic improvements which should enable us to make another small step forward.”