When his brother died, he felt anxious because he didn’t know what to do to help him. This sparked something in him that made him feel the need to learn everything there is to know about anything medical.
Dr. Glassman was his only father figure & he was a surgeon so that probably influenced him a lot, especially since his actual father was a no-good alcoholic that abused him
His brother always told him how smart and special he was and that he could do anything so he basically became a surgeon in memory of his brother
The fact that he could never help his mom escape his dads abuse probably gave him the deep desire to help people in any/every way he can
Finally, he became a surgeon because he is literally a genius. He has such a vast knowledge of the human body and what to do to save lives that he was able to be taken seriously despite his autism