Time constraints often require a pup to be left alone during working hours. The pup should be left where it can't do itself any harm. Leaving it in a run should be fine. This may cause some confusion in the house breaking department should you be planning to make him a house dog too, but he will probably learn as he matures a bit. The most important thing to any pups development however is socialization, so be sure to spend plenty of time with your pup in the morning and evening.