Help with a transgender friend

One of my friends that i've known since grade school just came out as FTM transgender to me. I will of course support him every step of the way, but i'm terrified of saying the wrong thing to him. I've slipped up twice now and called him a 'her', and i feel bad about it every time. He tells me that it's okay and he understands that it's going to take some adjustment, but i don't want to offend him. And i'm also just confused on what to say to him. I'm trying to hard to not offend him that i'm sensoring everything I say. He's going with the name Michael (pretty close to his previous name, Michelle) but i'm so scared of slipping up so i just avoid using his name altogether. Is that rude of me?