If all you want is to simulate, then a behavioural approach might be more handy than using real curcuit elements, such as a MOS (which, as I see, and as the other answers say, you have trouble setting it right). Instead, you can use the voltage controlled switch.
The control voltage you're using for that MOS is using a zero rise/fall time, which can't be physically true (no instant rise/fall in nature), so LTspice sets it, by default, to 10% of the Ton. In this case, for Ton=5\$\mu\$s, Trise=Tfall=1\$\mu\$s, which not only gives you an awful transition time, but also extends the value of Ton, from 50% Trise to 50% Tfall, to Ton+(Trise+Tfall)/2=6\$\mu\$s.
Your other values, for LC filter, for example, are awfully chosen: 1H and 10nF? How did you calculate those? I'll skip the load, maybe you intended to see the open load output? Somehow I doubt it.
If you're using the default diode, it would help setting its parameters epsilon and revelsilon, which control the knee-region to be a simple quadratic approximation, thus avoiding the sharp transitions that can cause the solver to yield timestep too small errors due to the possible discontinuity that results in a sharp derivative around that point. Or, you can keep things simple and use the default, quasi-real model by simply setting .model d d Cjo=1p, which only adds a capacitance to the junction in order to improve comvergence, while discarding the ideal model.
Also, using startup and uic, both, is not redundant, but not recommended, either (unless there are strict specifications, are there?). In your case, you're starting a switching application, so neither are needed.
Here's a reworked version of what you need, with some recalculated values for a 6.5V@1A output:
I fully agree with @Dave Tweed, too often people rush to blame the tool rather than the user.
Here's what a quasi-real setup would be:
Note the PMOS (usually when in high side), driven by a signal that's between GND and 13V (or close, some losses simulated) -- as the other answers have suggested --, and some parasitics. Instead of V2 there's usually some sort of control, voltage- or current-mode, that's a different question, but one that you should be able to find already answered.
