The adjective "BUONO" ( it is masculine and singular ) becomes BUON when it is placed before a noun. It isn't important if the noun starts with a consonant or a vowel ( e.g., BUON VIAGGIO, BUON AMICO, BUON FRATELLO, BUON UOMO, BUON PADRE, BUON NATALE, BUON ANNO), but there are some EXCEPTIONS.
In fact BUONO doesn't change if the noun starts with
- S+ another consonant ( BUONO STUDIOSO )
GN- ( BUONO GNOMO)
PS- (BUONO PSICOLOGO)
PN (BUONO PNEUMOLOGO)
z ( BUONO ZIO)
Y- (BUONO YOGURT)
X- (BUONO XILOFONO)
I hope my explanation can be useful.