Your brother-in-law's son is your spouse's nephew. While you remain married to you spouse, he is also your nephew, for he will refer to you as "aunt and uncle."
There is no genealogical relationship. He is just your son-in-law's brother.
Nephew
If your brother adopted you, you are his son or daughter and his in-laws are your grandparents by adoption.
Your son's wife is your daughter-in-law. Her brother is your son's brother-in-law, but the English language has no word for a relationship between you and him. You are not considered to be related to him.
Stepson; Son in law; Brother?
Your son's wife is your daughter-in-law. Her brother is your son's brother-in-law, but the English language has no word for a relationship between you and him. You are not considered to be related to him.
Your son's brother in law is likely your daughter's husband. If that's the case he's your son in law.
your son and your brother's son will be cousins
your son is your brother's nephew, and your brother is your son's uncle
your son and your brother's son will be cousins
your son and your brother's son will be cousins
Your mother's son is your brother or your half brother.