It's okay to be freaking out right now - you're going through a HUGE life change, so, naturally the insecurities are going to come back in. But here's my 2 cents worth:
As far as the stuff with his ex goes, from what I've read in this post, he has given you no reason not to trust him. If he has never lied to you in the past, why would he start now? I think you can take his word for it that he's not seeing his ex at all.
As far as being overwhelmed by his family, that is a valid concern, though I don't think that it'll happen. And dealing with those family members who aren't quite on board with what you guys are doing yet may be the reason why you weren't invited to his birthday stuff - If they're not going to embrace this the way you guys and your families have, then perhaps he doesn't want to subject you to that.
The truth is, his family is going to have to accept this eventually - this is happening whether THEY like it or not, and if you guys are happy and excited, then they should do what a family is SUPPOSED to do and be happy and excited FOR you guys.
As far as finding you less attractive - BAH HUMBUG! If he loves you, and loves the fact that you're having a baby, there's really no way he'll find you unattractive as a result. Especially since, from what I can tell, you look like you're going to be one of those adorable pregnant women you see. My cousin is one of those - I know what i'm talking about. Plus, there's that whole boobs getting bigger thing you'd mentioned in another post....
I hope some of that helps you a little bit. You're going to be an awesome mom!