il joue de la guitar. Why can't you omit the "de"?
In France, they do like this with all instruments.
It is the charm of a language to learn differences.
Il joue du piano, de la flûte, de l'accordéon, etc.
Why can you in english omit the preposition ?☺
In America we say "I am in the hospital" while in Britain they say "I am in hospital". The "the" is not needed.