I don't believe any form of chest exercise will do any good to reduce the size of breasts in men.
The reason some men develop larger breasts is because of a lower testostosterone level or higher estrogen levels.
There are certain foods/herbs which promote estrogen levels (for example saw palmetto contains certain saponins which when taken in quantity will enlarge mens breasts)
i think you need to investigate herbs/supplements that promote higher testosterone levels.
For example: damiana, sarsaparilla, dhea etc. Etc. Do some research on the topic or consult your doctor about it. No sense in keeping a low self esteem when the problem can be corrected.
One other thing you can have him try is high intensity aerobic workouts.
If he works out (for example jogging) he will notice a difference because prolonged exercise will increase testosterone levels. As you can see, some women who exercise too much will build muscles and lose their breasts. The same applies to men.
Best of luck to you!