Your nephew's son is commonly referred to as your grandnephew. Since you have the same bloodline, he is a blood relative to you as long as he is your sibling's offspring.
It follows that your grandnephew is your sibling's grandson, as well as your parents' great-grandson. If the child comes from a half-nephew of yours, then he is your half-great-nephew. There is often confusion between the terms "grandnephew" and "great-nephew," but they both have the same meaning. The difference lies only in the specific regions or cultures that use the term, but there is no difference in the actual definition.