+1 on the Hobbit. It’s an introduction to Lord of the Rings, films and roleplaying games.
Also the Discworld series by Pratchett is indeed safe for 7 year olds.
Not quite a series per se, but the 12 Heinlein juveniles might be something he’d get into. Some of them (the early ones in particular) have a bit of a '50s sci-fi vibe to them (TVTropes calls this Zeerust), but the later ones are quite meaty.