You cannot claim your son's investment loss on your tax return. The loss is his, not yours.
Even if he is not otherwise required to file, he should file a tax return showing the investment sales and the loss. He can then carry the capital loss forward from year to year. When he eventually has some income, the capital loss carryover will be used to offset some of his income.
When he files his tax return, make sure he checks the box that says that someone can claim him as a dependent.
