Transgender support

So just a little background my husband just found out he has a half brother. He let us know he is transgender and is starting the process to change. He just started the medicine. And will go through surgeries eventually. I'm fairly new to all of this. What is the process like going from a male to a female? While he's transitioning is it proper to say he or she when referring to him? I'm completely unsure so any advice would be amazing! Thank you in advance