If your tree does not show a graft and didn't have a tag, then it is probably a seedling. In contrast to what the seller told your father, most red Japanese maples develop the best color (that is, deepest red) in sunnier locations. Morning and/or midday sun is best, but afternoon sun can burn the leaves on some trees, particularly if newly planted or not watered enough. Seedlings can make very nice trees, but you never know what you are going to get. It might have very nice red leaves, or they might become more greenish with age or as the summer progresses. It is also hard to predict the ultimate height and width. I bought a red seedling years ago that kept its red color very well in summers, but it grew very tall and lanky. I've got a Bloodgood seedling that somebody gave me this spring and it's keeping the red color pretty well so far, but I wouldn't be surprised if it turns green later this summer.