What is the expiation for someone who swore falsely on the Qur’an under his father’s coercion when he was seventeen years old, and will Allah forgive him, and is it permissible for the father to coerce his son into that?

You have committed a great sin by taking your stepmother's money, for this is theft, and theft is one of the major sins. You must repent to Allah and return the money to her, even if indirectly.

Furthermore, your false oath denying the taking is another major sin, a "Yamin (definition) Ghamoos" (a false oath taken to usurp another's right) which incurs Allah's wrath and curse.

As for your father forcing you to swear an oath, this is impermissible, as the right to administer an oath belongs to the judge.