As far as weight is concerned, it isn't what you feed that matters --
it's how much.  If he is maintaining his weight on the amount you are
giving him now, you need to reduce that amount by ten percent.  Measure
carefully.  Weigh him on a baby scale every few days (under the same
conditions) and if he hasn't dropped a few ounces after a couple weeks,
reduce again.  Don't wait months.

When he complains about how you are starving him, just think to yourself
that complaining is good exercise.  And speaking of exercise, find every
little way you can to increase that --  more play times, more
interesting things to watch, etc.  Some people even try ways to make a
cat work harder for his food -- by putting his food somewhere where he
has to climb to get it, by tossing his kibble and letting him run it
down, etc.

Your vet's recommendation of the Science Diet M/D may not mean much.
Many vets are not really well-versed in dietary nuances.  Still, Hill's
generally know what they are doing with prescription formulas.  I don't
have any knowledge of the M/D specifically, but I assume it must be low
in carbs, which is the big thing for a DM kitty.