His concern over letting you drive it may relate to insurance coverage, or him being uncertain of your driving skills, a combination of both...but, who knows.
As Lantley noted, let him know you're a serious buyer and then it's up to him.
Make him aware that driving it is a condition of sale. Although, if you ride up front with him at the wheel, surely you'll notice if there are any handling type issues by how he handles the controls/steering wheel. Ask him to make a quick stop as well as full throttle acceleration. Also, consider making him aware that you would like to have the unit inspected.
I don't know what "safety" entails but, maybe you can factor that in to your offer.
Good luck,
Gary
