A radiologist should be able to identify an enlarged heart from a chest xray and you have posted his chest xray was fine.
Swimmers lungs often are large so I would not panic over the results of the Xray at this point.
Perhaps sitting down and talking through your bf regarding his known medical issues would be helpful. If he has high blood pressure is he on any kind of treatment? Also if he has high anxiety perhaps seeing a therapist to help him learn how to reduce his anxiety would be wise.
He is lucky to have a caring and loving fiancée.