25 month old boy started singing clical music?

Encourage it as much as you can! I know the symphony orchestra where I am occasionally does what they call "baby proms" which happen during the day, and are comprised of child-friendly music and pitched very much at kids; nobody minds if the children sing along! If you can find something like that near where you are then that's a great step, if not, why not suggest it to local orchestras? You've said you don't want to push him, which is fantastic; maybe when he reaches 4 or 5, see if he's interested in learning the recorder, which is an excellent starting point for any other wind or br instrument. Kids as young as 3 can also learn the violin through the Suzuki method, if he (and you!) are really keen. Meanwhile, keep playing him clical music - if he likes Chopin and Liszt, maybe try him with Dvorak and some of the Russian Romantics - and also, sing with him. The voice is an instrument that doesn't require any coordination and if you can get him singing in tune well now, you've formed a great basis for any instrument later in life.