Tough question. I don’t think any of us is informed enough to really help, because the issue here does not seem to be your relationship with your ex, as much as the relationship with your kid. I also have problems (much bigger ones actually), but at least my kid is 100% on my side, so I don’t have to worry about that bit.
So let’s forget about the ex for a bit and imagine that she’s reasonable (and even if she isn’t, you can cross that bridge when you come to it). The real risk here is that you force your son to do something he doesn’t want to (even if the fear is projected) and that in the long run he’ll just kick. Exactly because he is 11, he is already at that age where he can more or less decide what he wants to do, and even if you and your ex sort something out, if he doesn’t want to come, he just won’t (maybe not this time, but the next one and the one after that). So you have to somehow persuade him to overcome his fear and make the trip regularly in the years to come.
One way of doing this is promising him he’ll have a great time in the US, which of course for an 11-year-old is an exciting playground where everything is possible. I’m sure there are plenty of places he’d like to visit, and if you can make his stay there interesting, perhaps he’ll overcome his fear and come.
But of course there’s always the possibility that he won’t. Maybe it’s real fear, maybe it’s projected, but the point is that you have to make him overcome it (or keep flying there yourself, or move to Europe somewhere). Someone suggested flying over and fetching him back, which is actually a good idea (you only have to do this once, and hopefully there will be cheaper solutions in the future).
Another option is of course to go to Germany, but not because your ex says so. Not even because of his fear. But to be his dad, keep the connection, and give him a strong enough reason to overcome his fear in the years to come, when he’ll be making more decisions for himself. If I were in the US and my daughter was in Germany, she’d swim across the Atlantic to come see me, and I’d do the same for her. And no evil bitch is going to tell her otherwise.