Can i leave him at home

Question
My question is this. I have recently bought a 12 weeks old German Shepherd. I have only come to see that he is experimenting with my sisters staffy. Now her Staffy is 2 years old and he is only 12 weeks i am just wonderinf if im able to leave him at home or by him mounting her now will this cause a pregnancy?

Answer
Not for now.  He is much too young to be producing sperm.  Dogs are seldom able to sire a litter before 6 months.  Even then she can only become pregnant at the right point in her reproductive cycle.  The second and third weeks are when the most risk of accidental breeding is.

There is no good reason to keep a pet intact.  If she is spayed, that will end all worries about an unwanted pregnancy.  It also ends the mess, hassle and risks of her reproductive cycles.  Don't take this lightly. She will do anything to get loose and get to a male. Once loose, an accidental breeding and an unwanted litter is far from the worst that can happen. Such breedings can also result in fatal infections. She can get killed when she dashes heedlessly across the road in front of a car. Other dogs or coyotes may kill her.

He should be neutered by 6 months.  It will help with many behavior problems, and is healthier for him in the long run.