If I had a king who sits the throne by right of conquest. He has no natural born sons, but he does have a living father and a living younger brother. This is obviously throwing a wrench in my story because the father is important. My king is going to meet an untimely end. Who would be his proper heir in a patrilineal system? His brother or father?
Depends on the laws
There are legal systems in which inheritance can not go to the older generation, but not many. Typically inheritance would go to the father, then to the brother.
Because your character is king, he can probably decree who will be king after him in events of his death without heirs.
Given that he seized the throne himself, the laws may not matter. It may matter whether the father or the brother has the most support among the warriors.
"Precedence went initially to a dead person's issue descendants, in preference to collaterals such as siblings or aunts. Within the same generation of descendants, men were favoured over women, but only the eldest male inherited."
So it seems the Order is:
Descendents > Brothers/Sisters > Father/Mother > Uncles/Aunts > Grandfather/Grandmother
However - in a right of conquest system, the Father would be well within his rights to challenge his son to Combat to determine the winner.