Just to give you some background: I'm 9w 3d. My DH is 26 and I'm 24. We have been married for 2.5 years. He was married b4 me and though he tried hard the ex is beyond crazy. She moved almost two years ago now (very far away!) We get my step-son every summer (wish it was much much more), he is going to be 5 this month.
Anyway, we haven't told him yet because 1. Its early still and 2. We are afraid that the ex will do something and not let us have him for the summer, or just be very negative to him about the fact that his daddy is having another baby. We of course want it to be nothing but possative and we want him to know that we love him just as much as the one on the way. Making sure that she doesn't find out is hard though, keeping it off of social media until June or July is going to suck! I wanna shout it from the rooftops but if it is better for him then I will do whatever it takes to make sure she doesn't know until he is out here with us. Its just annoying. DH is so worried that his son will feel left out or replaced

Anyone else going thru anything similar? Any advice or encouragement?
Re: Any other FTM with a step-child?
I know it's harder for you because your little guy is with his mom the majority of the time. If I were in your situation with our little guy's mom I would definitely not say anything until he is there with you guys, so that you can be the ones to explain the situation, tell him how much you love him and let it sink in for him while he's with you. You DO NOT want his mom to be the one to twist the situation and turn it into something negative before you have a chance to explain everything to him from the happy place it should be coming from.
I feel for your situation though! That first year with my boyfriend was so hard. Baby mama up and moved to Texas with a boyfriend without telling us and he didn't get to see or talk to Celton for two months! If you have it written in paper, I don't think she can keep him away from your for the summer without plausible reason. As for her being negative just remind him how much he's loved and talk to him on the phone a lot about the baby and being a big brother, maybe even try skype, though that can be hard with an uncooperative mom.
Edited**
LOL i must say there's never a dull moment with him around. He keeps me busy && is apparently getting me prepared.!