I'm not trying to defend what your bfs dad said, as it sounds very cruel. I can, however, possibly see some of the reasoning behind it. I went out with someone who suffered mental health problems (depression, leading to self harming and suicide attempts) for two years. My parents knew about this, and although they never said anything as blunt as your bfs dad, they did make it clear to me that they didn't think I should go out with him. At the time I resented this, although I understand now that it was because they were scared of how it would affect me if he did anything stupid (indeed, when he sunk into bouts of depression, it would get me very upset, it was particularly bad during my AS levels, which my parents were furious about.) My dad has also suffered from depression in the past, and been on medication for it, so it wasn't a case of them not understanding, more that they were trying to protect me.
Obviously I'm not saying you'd do anything that would hurt/worry your bf, necessarily, but I expect his dad might well be nervous about this, especially if he doesn't understand the condition.