He is just a bathroom singer who likes to sing in the shower.
bathroom singer
idiomA person with mediocre or amateur singing capabilities.
Origin
disputed originAcoustically, bathrooms tend to give a false impression that one's singing voice sounds good, when in a more ordinary, drier acoustic environment it would be less pleasing.
Although he loves music, he is only a bathroom singer and never performs for an audience.
Despite his enthusiastic performances in the privacy of his home, he remained a quintessential bathroom singer, lacking the technical skill required for a professional stage.