No, you can't. It was his car, not yours; you have no legal right to it at this time, even if you would eventually inherit it. Further, you would not be a covered driver, so if in an accident, would not have insurance. To have legal authority over any of his "estate" (the assets, belongings, and money he left behind), you have to be appointed by the court as the estate's executor (if he had a will naming you to this position) or as the estate's personal representative/administrator (either term may be used). Once appointed to one of these roles, you would have the legal right and authority to drive his car. Contact the probate court in the county where he lived: ask how you can be confirmed as the executor (if there was a will naming you executor) or appointed as personal representive/administrator.