If he came from a reputable store or breeder, theres no reason not to believe them. It's kind of hard to imagine in this day and age for there to still be places that sell birds that young they aren't sure of the species on. At 6 months, he is a newly weaned baby and he looks it. Each individual bird has their own unique colorations, and with zons, the color spreads as they age, so he may very well get some yellow on his neck/back after his first molt. That all said, he just doesn't look like a yellow nape to me. The beak is not the typical mostly black color (that could just be the lighting, why it looks so light) and he would be an awfully small YNA (typical 14/15 inches 400-500 grams, larger individuals can be up to 600). They are BIG amazons. It is possible he is a hybrid too, though pretty uncommon, they are out there.
At the end of the day, does it really matter? Unless you are planning on breeding him at some point, any of the 'potential' species he could be are great birds. If it was me personally, I would assume a yellow nape because that was what I was told, and focus more on providing him excellent care and creating a lifelong bond with him.