As for those ??? novels, I haven't read the German versions myself but they started out in America as Alfred Hitchcock's The Three Investigators (not written by him, though) ... far more have been written & published in German than in English, though, as they're extremely popular in Germany.
I enjoyed the books (in English) as a younger teenager ...
I'll have to say Douglas Adams here - not just the Hitchhiker's Guide to the Galaxy set, but also the Dirk Gently books - where the "Holistic Detective" gets tangled up with mysterious & supernatural stuff on a regular basis.
Terry Pratchett is also excellent - again, not just for his better known Discworld series, but also the lesser-known works ( The Dark Side of the Sun , The Bromeliad (a trilogy about the Nomes, who live in the modern world but are only four inches tall) and some other stuff).
Garth Nix is a good author - not your 'standard' fantasy, either, but a cross-over - where magic meets (in one series) the contemporary world and (in another) a steam-level technology ...
Some military SF (with guts & gore): David Drake & S.M. Stirling have a good series, Hammer's Slammers about a hovertank mercenary company; Drake also did the Northworld trilogy, basically a high-tech melange of several ancient mythologies (including but not limited to Norse & Greek). Stirling has done heaps of other stuff, both alone and in combination with various authors (Drake, of course, but also with Raymond E. Feist, James 'Scotty' Doohan, Anne McCaffrey, Jerry Pournelle and Greg Bear).
Kevin J. Anderson is also a good author - has done a lot of SF, and has also written a lot of novels in the Star Wars universe (is your son interested in that at all?). He helped Brian Herbert (Frank's son) continue the Dune series as well ...
David Brin has written some good stuff, particularly his Uplift novels - based on a where humans have 'raised' dolphins and chimps to "intelligence", and also encountered an interstellar federation of races that see humans as rather barbaric. The Practice Effect is also a pretty awesome book.
btw, if you've done Harry Potter, I wouldn't count that as doing fantasy ... HP is to fantasy what maccas is to a lifelong career ... something to fill in time before you really get into it ...
Oh, last-minute addition - don't discount the classic SF authors: Frank Herbert, Arthur C Clarke, E.E. 'Doc' Smith, Edgar Rice Burroughs, Jules Verne ... some of the older ones are available as free eBooks to download, legally, as the copyright has expired without being renewed.