You will GAIN ke by lossing the mass
Theory is great, but reality is better.
Bows transfer their energy more efficiently to a heavier arrow. A heavier arrow also is less affected by wind resistance, therefore downrange the lighter arrow is losing it's K.E. at a faster rate. Add in the fact that heavier arrows have greater momentum which is more important once the arrow has hit hide, muscle and bone that can and will deflect it's energy easier with a lighter arrow. Add all this up and you will get many hunters defending the advantages of shooting a heavier arrow.
All that aside, with his set-up, K.E. is not a problem at any arrow weight. What he needs is an arrow that ends up where he's aiming. I suggest that this will be more easily accomplished by concentrating on proper tuning and making sure the F.O.C. is high (I'd shoot for at least 11-12% for most people). Picking an arrow because it's light is the wrong way to go about choosing a good bowhunting set-up. Out of that bow, he can shoot a log and be shooting plenty "flat" out to 30 yards.
I think that the average hunter will be more successful by concentrating on acheiving a high FOC while making sure the arrow is stiff enough to handle the extra tip weight at his arrow length and draw weight. Throw some helical fletching into the formula and you have a deer killing arrow. Speed does not kill. A sharp broadhead hitting a vital area with a least the minimum amount of energy and momentum to penetrate to the vitals, is what kills.