Get to know him. Have him send you several pictures of him, ask him to tell more about himself, what he likes and dislikes, have him tell you some stories from his life experience. At the same time tell him more about yourself, try to give him a truthful picture of you. And be sincere with him. This way you can expect him to be sincere too.
Insincerity is very obvious in letters, even more obvious than in a conversation.
Another good side of building relationship through correspondence is that you can be sure about man's intentions. Those who just want to play games won't write letters for several months. Don't make him wait too long either. Even the most serious gentleman may lose his patience. I am not a lawyer myself and cannot give you a professional advise on immigration matters.