How do I record this in Family Search
FamilySearch allows you to change his/her sex. Since he/she is still living you would have to create him/her. Also remember because he/she is living only you will be able to see any information or explanation you add until he dies and you mark him deceased. If he outlives you what you create will not be seen until 110 years after his/her birth year. If you want the information about the sex change operation attached to his page I would add it as a discussion. Only discussions cannot be deleted by another individual.