Welcome to the forum Mark,
"he is onto something" is an affirmative idiom meaning the person has just said something which opens a new way of understanding or dealing with a problem.
To say this of one's self, in the first person, is to miss out the affirmative action. It implies you are very confident about your action. We would usually modify the phrase in the first person to:
"I might be onto something" - this prevents us sounding over confident or dismissive towards the person who has asked us what we are doing. It leaves the subject open to further inquiry, inviting affirmation from the inquirer.