What is the best method to desensitize a dog?

I've had my dog for a little over a year now, I got him from the local pound and it was pretty obvious that he was abused by his former owner and because of that he is "terrified" of my boyfriend. Is there anything we can try to desensitize him to help him get use to my bf? He's such a great dog and it's really hard to see him walk on eggshells around the house when my bf is around.