As I understand it:
the reflexive pronoun is used when the object of a sentence relates to the subject e.g. puer cor suum sequitur - the boy follows his (own) heart.
to convey a meaning of obligation, latin uses gerundive + esse construction. Since this is passive, the subject in latin is what would be the object in english, and the thing that is being obliged takes the dative e.g. pons puero transendus est - the bridge is to be crossed by the boy i.e. the boy must cross the bridge.
I am now getting a bit confused if I combine these two principles. How would I say something like "the boy must follow his (own) heart"? Is "cor suum puero sequendum est" correct, since cor suum is the subject, can I use suum (what is it reflexive of)? If not, how else can I write this? Could I change puero to puero ipsi?
Any help/advice is greatly appreciated!