No, you  cannot will another person's property, and if the ATV is in your name, not his, it is your property: his will has no power over you or ATV. Wills are typically written long before people pass away, and so when the write them, they don't know what they will or will not own at death. Therefore, the way to really read or understand a will is as a set of "contingencies"; i.e. "IF I own the following ATV, it should be sold to pay for funeral expenses." Since he no longer owned it, it no longer falls under his will.