Here's the research I've already done:

He's a snub nosed dog so he's not allowed underneath the planes in cargo due to risk of death. But he's freaking dense so he's not allowed in most cabins because they have a 15 lb weight limit.

United Airlines seems to have a size limit for dogs in the cabin, but no weight limit. However, both on the website and the app, when I search for transatlantic flights, I get error messages.

I also researched some cargo pet transport companies but I'd be looking at $3k each direction.

Our starting city is San Diego and our destination is Formentera, Spain. But if we could find a way to transport him from San Diego to anywhere in Spain or France, we could go the rest of the way by train so that would work just fine.