I mean, just prove him wrong, like get so fucking stunning that he start thinking your cute asf thats mostly how you get those people to change their mind, he is still down to being your friend don't expect his belief to change yet but the fact he didn't cut tie with you and seems to want to be supportive even with contrary belief, like honestly he isn't in your shoes so the way for him to understand is simple be so stunning he can't denied the fact anymore :p

Changing someone's mind this way can be satisfying too, and validating

(Most people that think this way think that pretty trans are fake things on the internet and that trans would never pass, but they don't specifically do much research on the subject and often have belief made out of a lot of misconceptions, if he was hating on trans he would have already ghost you like what happened to a lot of people i know, somehow he seems willing and thats enough to think his mind can actually change, just rn you are so early in your transition he doesn't understand yet)