The ratio of gravitational force on Neptune to the gravitational force on earth is given as 9:8.

So his weight on Neptune is

\frac{9}{8} \times his weight on Earth.

= \frac{9}{8} \times 792 N = 891 N

I found an answer from downloads.cs.stanford.edu

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I found an answer from nssdc.gsfc.nasa.gov

Planetary Fact Sheet - Ratio to Earth

Oct 21, 2019 ... MERCURY · VENUS · EARTH · MOON · MARS · JUPITER · SATURN · URANUS · NEPTUNE · PLUTO · Mass, 0.0553, 0.815, 1, 0.0123, 0.107 ...

For more information, see Planetary Fact Sheet - Ratio to Earth