You are getting confused over wrong issues here. It is not our domain to decide whether someone is a martyr in the eyes of God or not or how would God deal with someone who has already passed away. These matters are to be decided by God.
The good thing is that we have glad tidings for both minor and major sins.
Quran says that if a person is able to refrain from major sins during his lifetime, then its reward is that his minor sins will be forgiven by the Merciful Lord.
إِن تَجْتَنِبُواْ كَبَآئِرَ مَا تُنْهَوْنَ عَنْهُ نُكَفِّرْ عَنكُمْ سَيِّئَاتِكُمْ وَنُدْخِلْكُم مُّدْخَلاً كَرِيمًا
If you abstain from the major sins of what is being prohibited to you, We shall forgive your minor sins and shall make you enter a place of honour. (Quran 4:31)
Second glad tiding is for the major sins if committed by a good conscious muslim ever in his life, then death by such suffering/disease/accident becomes an immediate kaffara for those sins as decribed in the above reply.
You should also keep in mind that this world is created on the principle of ‘Ibtila‘ (tests and trials) which entails that every human being will be tested in one way or another in this world, some through a finanacial distress, some through health, some through matters of family etc., the list is endless indeed. And whoever shows patience and humility through these tests and is able to achieve the required Tazkiya, then that person will be successful on the day of judgement. Manner of death in this wolrd is also a part of this whole scheme of tests and trials. Therefore, your job as a son is to know that God always fulfils his promise and hope that He will shower his extra mercy on you father’s soul and you shoud keep praying for him instead of pondering over matters which you have no control of.