You’ve got 1 year, at the most, to instill delayed gratification & how to be a person of integrity in this young man before he turns 21 & is of age, legally, in the state of New York.
I’d recommend going to a frum counsellor immediately. Why? Simply because asking here in the Coffee Room shows that you don’t have the experience nor the tools in combating this problem professionally.
I am assuming that this individual lives at home. Why not make an enforceable rule that the computer(s) are placed in a public room, such as a kitchen, dining area, or living room, and forbid the use of computers in bedrooms and other private areas of the house or apartment.
Next, you should install a filter system on said computer(s) to prohibit visiting sites which are treif.
This doesn’t prevent everything that should be blocked being blocked, such as the “Dating Without Tachlis” group on Facebook, for example, so you might want to install a keystroke logger program and let it be known to the 20 year old all software will track him.
He could end up going to internet cafes, public libraries, or using wi-fi on a smart phone or iPod Touch. He should not ever have a smart phone, iPod Touch, or tablet, simply because you know what WILL happen.
He has to be made to understand how pernicious this problem can become in his life and that every man has to deal with it. Dealing with it in a mentschlich way will make him a happier person for himself. The choices he makes now in his life will affect his future greatly.