Welcome to the site! Probably the "best" horror books we have here are the Harvey Books, but those can be a bit extreme even compared to the EC's, so I would check them first. In fact, just check everything first; I don't want to suggest a particular book and then have it be too extreme for a 10 year old. ACG is generally (but not always) a bit milder while still being firmly pre-code; and Strange Terrors from St. John has some Kubert art in the mix, which is always nice. I would avoid Charlton's The Thing; it has some great Ditko art but is notoriously gruesome even by pre-code standards.
Those would be my first suggestions in the horror department; although when dealing with pre-code horror for somebody of that age I repeat: check it first yourself!