Technically.....
Well, technically he IS right. It was, in fact, a truck. There are identifiable distinctions between a car and a truck. Focus on his "successes" in observation. Reward him with a genuine Nascar beer mug or a trip to wal-mart. If you love him, you will teach him to read and to put the toilet seat down by offering him rewards for seeing distinctions that would, I am sure, otherwise evade him when he is distracted by food and sex.