While everyone has made some great responses, what is getting lost in all of this is the fact that he is a Boyfriend. not a Husband or fiance.
If he was one of the latter 2, dad may feel that there is a deeper commitment between the 2 of you and the relationship would be less likely to crumble under the pressure of 2 people in 2 tough fields studying all the time and spending loads of time apart.
Maybe dad feel that if your boyfriend was also getting into medicine, he would better understand what you are going through and vice versa.
Dont worry. keep loving each other. Dad will come around eventually.