It is tricky - how to go without being seen to be condoning this exercise in manipulation. Giving a speech and being made to feel good about it is important to the process of indoctrination by linking ego to belief ( he feels good about the religion because he is made to feel good about giving the speech). If you do go, you need to sit him down and explain to him why it was difficult for you to go and that even though you didn't agree with him doing it or with what he was presenting, you went to support him. if you get shunned, make sure to let him understand that happened.
Interestingly, when the shoe is on the other foot even ex-Witnmesses have trouble doing what you propose but in reverse. Even though my wife has stopped going to meetings, when my son joined the choir at school and had to perform at the local Anglican church, my wife would not go to hear him sing. But this last weekend she came with us to La Sagrada Familia in Barcelona (which is a basilica) and so there is no consistency to her behaviour.