Some good advice on this thread, however I do agree to socialise the pup as much as possible, but PLEASE think about the way the dog sees the world and its temperament. Don't push the dog into too many situations where it is uncomfortable or will react. Read the dog, sus him out, take your time and start with small steps and slowly build his confidence, and don't push it to hard by dumping him in the deep end.
Just take it steady, take him out and about, and make sure you are letting the dog go at his own pace, but now at 5months is the time to do this, don't leave it too late.