Which is correct and which is colloquial or incorrect of the three choices?
I would certainly use "in", but what about the other options?
Interest in can refer to a desire to learn something or a desire to engage in an activity or behavior.
Their dog showed very little interest in food, so they knew something was wrong.
Has your child shown any interest in joining a sports team?
Does he have any interest in auto repair?
Interest to can refer to a claim of ownership in something.
She has an interest to a valuable property on Main Street.
Interest toward can refer to an intention or purpose.
The city council has raised the parking tax by 30% with an interest toward reducing traffic congestion downtown.
The usage of interest(ed) in and interest(ed) to was outlined by user Egghead99 in this helpful answer.
"Interested in" is used when what comes after it is a noun, or a verb acting like a noun (known as a gerund). [...] Therefore, this sentence usually takes the form "[Someone] is interested in [something]."
"Interested to" is used when what comes after it is a verb in its "to form" (known as an infinitive). [...] This type of sentence usually takes the form of "[Someone] is interested [to do something]."
Interest(ed) towards, however, is a bit tricky. It has consistently been unpopular and rarely used throughout all periods, according to Ngram. Therefore, it is not by any measures colloquial. In the few cases where it was used, the word toward does not seem to accompany interest itself, but rather other constructions, such as direct (towards) and turned (towards). In one case, interest toward seem to have been used exclusively to avoid repetition. In some other cases, like this and this, interest(ed) towards is used in some situations where it is effectively interchangeable with interest(ed) in.
To sum it up, you are interested in something, interested to do something, but most likely won't be interested towards something.
"Does your son express interest in music?" - this is the correct and most common option, a mere inquiry about your son's attitude.
"Does your son express interest toward music?" - as "toward" is the preposition showing direction of movement (not especially physical movement!), this expression is not grammatically incorrect, but it has a strong implication that your son hasn't been interested in music before (or is not expected to be interested in music).
"Does your son express interest to music?" - I believe, it isn't grammatically correct. Please, note, that I'm not a native English speaker.
