Well, technically your son is not a toddler, he'll be in pre-K next year so he's really a pre-schooler developmentally. A toddler wouldn't really do well traveling alone on a plane to visit or stay with dad. Older kids do travel like that but I can't imagine flying from Germany or London across the ocean to a major airport, change planes and perhaps have major layovers then get to another plane and fly to the middle of the USA then on to California. I would think it would take a couple of days and a kid wouldn't be able to manage that. So I don't see a judge granting over seas visitation in any way or any sort of custody since the judge wouldn't allow kiddo to fly alone and wouldn't make you spend that much money to take kiddo to see dad in his home land.
I don't know....really, dad's coming to see his kiddo and participating in his life. I think that's good especially since it's a really really long trip to get from there to here. Has he considered moving to California for a few months of the year and taking kiddo for extended periods of time? Is he wanting to take kiddo to meet his family and friends, to see where he lives?
I'd think he has that right if he takes kiddo with him and brings him back. I wouldn't let him take kiddo unless you knew for a fact he'd bring him back. That would be scary for me. What about him paying a ticket for you and kiddo to come there. If you have legal custody but you're in another country I don't know how much weight that would hold.
I do have a friend who goes to Finland every year to see her daughter who married a man from there. Life is so much better there, it is so different. They don't have juvenile delinquents. If she ever gets granted the ability to move there and be legal she is going in a heart beat.
Have you and he talked about this? Have you made an appointment with an attorney who knows international custody laws?