
An 8 month old staff has still a lot of training and growing up to do, and he needs you to concentrate on him to ensure he grows up a nice well adjusted boy. the worst is yeat to come with his teenage kevin stage only just begginning.
I would wait until he is two or three before getting another dog, byt which time you will be able to concentrate on new pups training, without worrying about the older one as much. also with a bigger age gap the older one will be established as higher in hierachy, so less likely to have problems between them when the younger one reaches maturity. As Stafs are often a bit dog dominant, you may be better off getting a bitch next time, and have one or other (or both) neutered!