And how will he know, with his altimeter stuck?
Let us, for the sake of the discussion, assume that it is instead the altimeter setting (Kohlsmann) mechanism which has stuck.
First off, he should descend to a height of 3,500 feet ASL. That is what they tell him to do. How to actually go about this will become a bit tricky though, as he cannot change his altimeter setting which means his altimeter will misread.
1 hPa is approx equal to 30' of altitude.
If you increase the reference pressure (Kohlsmann setting), the difference between your current (lower) ambient pressure and the reference pressure increases. Your indicated altitude increases.
In the example given, you will be flying at an altimeter setting of 1013 when you should have 1000. This means your altimeter will overread by (1013-1000)*30 feet, or 390 feet. Descend to 3500+390=3890 feet of altitude.
Another way to think about it is "altimeter setting down, altitude down". Picture yourself at your intended 3,500 feet at the correct QNH of 1000. How would you get to your actual QNH of 1013? You'd increase QNH - alas, increase the indicated altitude.
I thought I was doing good trying to avoid those airport hotels... and look at me now.