That is correct, a seller does not have to accept a cancellation, it messes their sales up, so they expect to be paid for the sold item.
You are obviously not going to do this, so you face an unpaid item case being issued to you after 4 days of non payment. You will then receive an unpaid item strike on your account, which lasts 12 months.
One will not do you much harm, but gain another, and you will not be able to buy from the majority of sellers for a full 12 months.
Everyone is responsible for their eBay accounts. I would strongly recommend changing your password so your son doe not access your account again, and fully log out when not in use.
@wilcr_6578