Heart disease can lead to shock if there was some acute episode, but I agree with Vash the whole thing sounds off. He's too young. Psychosomatic?
Perhaps by 'weak heart' she means 'nuts'?
Anyway, sound like he is way to sensitive for you. Your freedom is going to be seriously curtailed in a relationship like this. Oh, and did I mention 'mother in law from hell'?
I think you should consider using this period to break things off. Say you are sorry that you contributed to [whatever the hell is his problem] and that perhaps it would be better if you simply weren't around to contribute to his distress. Blah, blah, etc.
Good luck. Any relationship where you feel worse in it than out is not a good one.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh