Don't go back, go forward. I think in this case you'd be better off checking with a local arborist rather than Usenet replies -- arborist thinking in the U.S. opposed the use of concrete and fillers starting many years ago, because it interferes with the tree's own efforts to repair itself and is likely to lead to rotting wood and other problems. Local nurseries still sell stuff for filling in tree wounds, but that's a victory of marketing and out-of-date information over effectiveness.
Before you spend any money to "save" the tree, check with local arborists to see if this is a known problem species while it's still young and easy to replace. Your nursery may have an insecticide for the borer (that's what it sounds like you have) but you might also check with a local arborist and see if borers are a known problem with this type of tree. While it's young you might just want to replace it with a different variety, especially something that's a native tree, rather than keep trying to save a tree that's in the wrong region and in steady decline --
If you're at one of the American bases in Germany you may be able to find someone in Civil Engineering with local knowledge of trees and a list of problem species.