I would also be concerned that your son has not thought through or truly looked at the practicalities of what he wishes to do. From my understanding $500 is a drop in the ocean with respect to costs of transition financially. Does he have any idea how he is going to finance this? That sounds mercenary but I think it is an indicator of poorly thought through decision process, along with the reluctance to return to a psychiatrist.
Your son needs to recognise that you still love him even though you may disagree with his decisions. I think it is unfair to intimate the use of an emotional blackmail card (i.e. if you love me then you will do whatever I ask). He has responsibilitie to his family as well. When we are young these are not always on top of our mind and in that respect I can see and understand his position but I believe as a parent you have a right to be heard and understood, as does he.
Good luck. I agree that you must talk face to face. His statements regarding phone and email sound like avoidance in the same way he is avoiding WPATH guidelines.
Best wishes.
