If the English language were completely logical, this would be true. However, English is not a completely logical language and the term "great son" simply is not used.
Your son and your nephew, or niece, are first cousins to each other.Your son and your great nephew (the son of your nephew or niece) are first cousins, once removed, to each other.Your grandson and...
Your nephew is the son of your sibling and a first cousin of your children Your grand nephew (or great nephew) is the grandson of your sibling and a second cousin of your grandson Your great great...