It’s great that you found a good therapist and are working through things together. An important question to think about with the sleepover is: Would you say no if he was not transgender?
Think about what causes your discomfort around it. If he had been born a cis-male, would you say yes to the sleepover? These are all important questions to think about that may help you understand your feelings around it better. Talk to your son about it, too! Explain your discomfort and see how you can both come to an agreement or compromise so that you both feel comfortable and happy with the decision.