You can't offset the income with education expense. On the federal tax return there is no deduction for education expense below college level.
Your son's income is low enough that if the capital gain is long-term he will not have to pay any tax on it. It will be taxed at a 0% rate. You have to enter the long-term capital gain in his tax return, and it will be included in his income, but it will not increase his tax.
