buy earplugs for him :) Then its almost silent Model M ^^,
Show Image
(http://www.activevibrant.com/catalog/images/hearing/Reusable%20Ear%20Plug%201260.jpg)
Oh yeah, we forgot to mention the ear plugs mod. It has the best results so far, but the drawback is that the results are temporary and you need to re-apply the mod each time. You may also have to deal with ear crud, which is another drawback.
This is permanent, just apply once in each ear:
Show Image
(http://rideliquid.com/wp-content/uploads/2010/03/Air_Horn.jpg)
Bah, that mod is impossible to complete. You might be able to finish the job on one ear, but the second one is near impossible to do.