OK so you learned a lesson the hard way. Get your money back form him, then go to Z-Bart and have them power wash the whole underside along with the frame. Hopefully he didn't spray the inside of the frame! Then have them spray their rust inhibitor on the frame and under body (what you should have done in the first place) and after letting it sit two days, they will undercoat the truck.
Never Ever pour/spray oil on to or in to body panels. All it will do is attract dirt and then moisture causing RUST! I love hearing guys say "I'v been doing this all my life and never had a car/truck rust from it". That pure BS!
Don