The tax to transfer a silencer is $200, the same as a machine gun. Every buyer pays it, whether it's new or used. There's also some paperwork to fill out; the same paperwork you'd need to buy a machine gun.

They're illegal in Illinois (and possibly elsewhere) but legal federally. It would work better on a semi-automatic if you lock the slide.

.40 is also something to consider, if he thinks 9mm is too small. Bigger than you need does no good and limits the number of rounds that you can fit in a magazine. Of course, if you need more than two, something else is wrong.