As others have said, you can not claim his capital losses on your tax return.
I'm of the opinion that he does not need to actually file a tax return in order to carry forward the losses. But what he does need to do if accurately calculate the carry forward amount, which usually means at least preparing a tax return. If he has absolutely no other income, then the entire loss carries forward.
Another possible reason for him filing is the 1099-B issued by the broker. If it's large enough and does not show the cost basis, an IRS inquiry could be triggered.
Do I have to use a capital loss carry over carry forward?
Basically, you are allowed to use your standard or itemized deductions to reduce your taxable income before having to use your capital loss.
Technically, if your standard (or itemized) deduction is greater than your AGI, without the capital loss, then all your capital loss is carried forward. A dependent does not get a full ($12,950) standard deduction.
