Smart is good, but being confident in his clothes and not fidgeting in a collar and tie if he is not used to them are more important.
DS wore black jeans and a smartish jacket for his university interview/audition (music)
Whatever he wears needs to be clean, unstained, and ironed if it needs it - looking like you don't care about your appearance makes them worry if you care about other "boring" things like punctuality as well.
DS went for his 6th form college interview today wearing jeans, t-shirt and a zipped top. He had been due to wear school uniform, but school was closed due to the snow. I suggested smarter clothes, but DH thought 'student wear' was fine. DS told me there was another lad there for interview who was also wearing jeans and a girl in a very short skirt!
I am of the opinion that smart clothes should be worn for interviews/meetings of any sort - but sadly DH & DS don't always agree with me!