Either a step-nephew or a son.
"Step-nephew" is not a widely accepted or used relationship term but implies that someone's sibling has married a person who has a son from a previous marriage. That son is the step-son of the uncle's sibling, if the sibling is the person who is involved in the marriage. Otherwise the uncle has more than one sibling and you have another step-nephew relationship.
You brother-in-law is uncle to your children, so his great nephew could be the son of your child, also called your grandchild, or the grandson of one of your siblings who are not married to your brother-in-law, and thus your great nephew as well.
But your brother-in-law can also have great nephews who are not related to you at all.
if they have a relationship and they are both boys and love each other i mean like love love that means there gay but i not sure wut your question is i guess you ask this
First cousins
First cousins
Great nephew
Step-brothers
Second cousins once removed.
You and your first cousin's son are first cousins, once removed.
Your son and your brother's grandchild are first cousins once removed.
your son and your brother's son will be cousins
your son and your brother's son will be cousins
A father's first cousin and his son are first cousins, once removed. A father's second cousin and his son are second cousins, once removed.
your son and your brother's son will be cousins
Two people who have at least one grandparent in common are first cousins. Your son and your stepbrother's son may be first cousins, or may be unrelated. Even if they are unrelated that can call each other first cousins if they get on well together and want to do that.