I think your player is missing the point.
1) Gun powder for fireworks exists but clearly only to initiates so restricted or else the 'The Hobbit' etc would have many references to firework displays. The Noldor should have been employing it but perhaps wisdom has stopped them.
2) If a fireball is common then why does Gandalf choose to use small conjurations augment fire? For that matter, Saruman could cast the odd fireball but doesn't. Also, Sauron (a fire Maia) does not do so either. A debate on magic in Other Minds or Other Hands probably covers some of this.
3) The fire of Orthanc is some form of explosive but we never find out what (except that like many weapons of mass destruction, Tolkein despises it). Yes I know the films show black powder but the books mention nothing.
4) Even the Orcs (masters of horrible contraptions) have not mastered casting of metal to make artillery (canons and the like). Hence rifles and the like are also unavailable.
In short, if your players want a more Medieval setting, then turn to Warhammer Fantasy Settings to fulfil your wish to have a dwarf with a blunderbuss.