My friend recently adopted young rescue dog which is terrified of everyone and everything. He wants to rebuild this dog confidence. 

It looks like as though this dog has been inadequately socialised and/or mistreated. He must check his food drive due to this is the key to successful integration and training, although it will take long time and slow process. 

Use snack, treat with great smelly as reward and teach him by given all family members chance to give him the reward to learn him that people can be rewarding, and time passed will gain his trust in coming to family members.

 

After sometimes he show better behaviour carefully bring him to meet new friends, place and do the same and not expose the dog more than he can cope with. Advise to my friend that all job must be carefully, step by step, and patient. Normally most of the dog will gain his confidence back ..