telling boyfriends sons mom he's going to be a big brother

My boyfriend has a 5 year old son already and him and I are expecting our first together in April. We just started telling everyone and plan on telling his son this week and then his sons mom...he doesn't want me there when he tells his sons mom. He says it's rude if I come. Im kinda hurt by it rude? Anyone else been in this situation or anything similar? 
UPDATE...we told her and I was there...I was trying to concider her feelings the whole time...I didn't want her finding out from someone else. We told his son together and it went great..he doesn't fully understand he was just excited because we got him a batman shirt that said "big brother" after we told him we went over to her house...we said Joey has something he wants to show you and she said it was ok to come we got in the car and he put a sweatshirt on to hide his shirt...he was so excited it was cute...his mom said what do you have for me bud and he looked at us to see if it was the right timing to unzip his jacket lol...his moms jaw dropped then she came over and gave me a big hug and said congratulations. Later that night she texted me and said she's very happy for us and appreciates all I do for her son and knows I'll make a great mom...she even friend requested me on fb and posted a congrats on my wall. She said she was happy that the three of us told her together instead of finding out through someone else. Her and I are not best friends but I think this will bring us closer together and I'm glad I listened to my gut feeling on wanting to be there and not let my boyfriend talk me out of it