He would have to start by getting a Fiancée Visa, K-1.
Once he has that he can enter the US, you can get married and he can get a green card, it's not easy and in the end you'll probably have to pay well over $2000 for all the visas and other fees but if you can prove you actually have a pre-existing relationship you should be fine. They ask for a lot of proof of the relationship because they are always on the lookout for marriages of convenience, where the person overseas pays an American to get them a green card.
Spouse and Fiance(e) of an American Citizen