Vehicle ownership/insurance related questions

I am looking to insure my cousins vehicle with my insurance to get better rate as he hardly drives his vehicle. When talking with the insurance company it sounds like the vehicle will have to be in both our names for the insurance to work. Since he currently owns his vehicle (bought second hand in 2024). If we were to fill out a new bill of sale to include me as an owner so I can add my name onto the registration for his vehicle, would he fill out the sellers information? Or how would it work to write a new bill of sale when I’m not actually paying him for it we would be co-owners. Any advice would be appreciated!