9 year old yellow tinted teeth

My son is 9 , he brushes his teeth every night with Colgate . He has only ever had 1 cavity is whole life . Now his teeth always have a very slight yellowing . I’m not sure what’s causing it . I asked the dentist he said to just wait till he is 17-18 and whiten his teeth . But I don’t want him to go his whole childhood with a yellow tint . How can I improve his confidence by making his teeth whiter and getting him to smile more .