All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
Firstly, we implore Allaah to forgive your father, raise his rank in Paradise, illuminate and expand his grave for him, generously reward him for his good deeds, and pardon his bad deeds. We also supplicate Him to bless you with dutifulness towards your father, help you earn his pleasure, and admit you both to Al-Firdaws, the best and highest part of Paradise.
You should know that if you choose to carry on with your study in medicine school with the intention of fulfilling the wish of your father, you hopefully will be rewarded for such a good intention. Verily, the intention has an impact on the good deed and earning rewards. This is why one of the righteous predecessors said, "Man's intention far reaching than his action."
You should also know that it might be recommended for you to quit medicine school if you absolutely dislike it, especially if it would bring the illness that you mentioned in the question upon you. You can still earn rewards through supplicating Allaah, The Exalted, for your father to a good effect, showing kindness to him through offering charity on his behalf (and dedicating the rewards to him), honoring his friends and showing hospitality to them, and upholding ties of kinship with his relatives. Verily, all these acts fall under the category of dutifulness towards the father that does not end by his death.
Abu Usayd Maalik ibn Rabeeʻah As-Saaʻidi said:
"While we were with the Prophet a man from the Banu Salamah came to him and said, 'O Messenger of Allaah, is there any way of honoring my parents that I can still do for them after they die?' He said: 'Yes, to pray for them, to supplicate for their forgiveness, to fulfill their promises after their death, to maintain the ties of kinship which cannot be maintained except through them, and honour their friends.'" [Abu Daawood]
Allaah knows best.