The exact change in the gravitional acceleration is rather complex to calculate but taking the simplistic 1944 Rubber Bible figure of:
-3.08600E-004 cm/sec2/m or
-3.08600E-006 m/sec2/m
for a mass of 100Kg and extending it to 16,940 m
we get -5.22768E-002 m/sec2/m or a reduction in his weight of, 5.23N due to the altitude. If he was over Halifax (Nova Scotia) at the time this would reduce his weight from 980.574N on the ground to 975.346N. if however he was over Bahia (Brazil) this would reduce his weight of 978.331N to 973.103N.
If I was sober I would use the more suitable methods given in Applied Geophysics (Telford et al)