You may never know the past of your new companion, but that’s okay. His new life starts the day you bring him home. What he needs most is for you to limit his stress, while increasing structure in the form of routines, training and confinement. Offering daily stress-release outlets, in the form of play, exercise, toys and positive training also helps.
Limit Stress
Whenever possible, bring your new puppy home during a relatively calm time (not during the holidays, for example). Keep things as quiet as you can. The first few days are not the time to introduce him to the Boy Scout Troop or all your neighbors. Remember, he has no clue as to who you are or how you will behave. Give him time to settle in to his new life.
On a more basic level, feed him whatever he has been eating. If you don't know his diet, choose a premium brand puppy food and mix it half and half with cooked white rice (not the instant kind). This will limit diarrhea, which is something both he and you will appreciate.
Structure
Being social animals, dogs fare best when there is order in their group. Order is demonstrated through clear, calm leadership. Simple things really - having him "Sit" before you open the door, having him "Wait" when you let him out of his crate, practicing "Leave It" a few minutes each day. Makes these special fun times between the two of you. Smile, laugh, be pleased with every tiny effort he makes. These little things go a long way to making your new puppy feel secure and comfortable as well as building his trust in you.
Routine
Few things help an anxious dog like a routine. The more routine, the better. A biscuit before bed, a walk first thing in the morning, a little romp during commercials - make up routines. Figure out ways you can make his world make sense to him; he'd thank you if he could.
Training
We say training, but what we mean is building trust, forming a relationship, creating communication. Again, what you teach him to do is less important than how you teach him to do it. Teach him tricks, make him a pet therapy dog, work on agility. Do what thrills you both. Think, "This animal wants to please me; it is my job to show him how." And when he doesn't, have faith that it is because he does not understand, not that he is being difficult. Such an attitude is an enormous gift to anyone, but particularly sweet to a dog who may have had much disappointment and confusion in his life already.
Confinement
Give him a room of his own. Many adopted dogs thrive on a confinement schedule partly because it limits their decisions. When in the crate, all is well. Crating also prevents stress-related behaviors like chewing or housebreaking mistakes.
Play & Exercise & Toys
Joy is an important addition to a dog's life. Cultivate it wherever and whenever you can. Daily exercise and romps go a long way toward lessening stress, building attachment and having a happy, well-behaved dog.