See an immigration lawyer, alone, to learn more about the process and what you are getting yourself into. The most important thing you need to know is that if you sponsor someone for residency, you will have to sign an affadavit of support, which means you will be financially responsible for him. This means that if you get married and he gets a green card and racks up 50k in credit card debt, you are responsible for paying it off he he can't or won't. You really really need to be certain you can trust this person because it does happen that marriages break up, the green card holder gets into financial trouble and bails, and the sponsor gets stuck with the bill.

There are other important things to consider as well but in my opinion that is the most salient.