Who is my aunt to my child?
Considering this, who is my uncle to my son?
Your uncle is your son's grand uncle (paternal) or great uncle (maternal). Your son is his grand nephew.
In this regard, who is my cousin to my child?
While from a genealogy standpoint, your cousin's child is your first cousin once removed, but the common name to call them is niece or nephew. They would call you aunt or uncle, and your children would simply call them cousins… although of course, they are really second cousins.
Your great-nephew is NOT "your son's brother's son" (i.e., your son's nephew), but rather it's YOUR nephew's son. Your great-nephew is the grandson of YOUR brother or sister. your great-nephew is your son's first cousin once removed. Your great-grandnephew is your sons first cousin twice removed.