I'd still get professional advice, but I think so-signing the loan just makes loan responsibility fall to him if you don't pay. I'm not sure if that would allow him to qualify to receive the federal tax credit since he won't be the one purchasing the car. While you can probably have both of your names on the registration, it's the title that comes with all the financial responsibility. To be honest, I have no idea how having joint ownership of the title affects the the tax credit, but I do believe at least he has to be on the title in the purchase in order to qualify. If he's on the title (whether you are or not), complications follow in trying to transfer full ownership to you.