i saw a tv programme on this a while ago, and as harsh as this may sound, you should pay little attention to the dog for a while and let your boyfriend do the things you would usually do. This could make him feel that there are people besides you who can care for him. However if the dog is still scared and refuses to bond with him, don't drag it out too long. It could easily go the other way and make the dog feel like you have also left him.

Hope they get on soon!