You cannot report your son's stock sales on your tax return. It doesn't matter whether he has a loss or a gain. You have to file a separate tax return for your son.
You can include his interest and dividends on your tax return, but not capital gains or losses.
It's generally better to file a separate tax return for the child, even if his income is only interest and dividends that could be reported on your tax return. The only advantage of reporting the income on the parent's return is that it's less paperwork. It will not save any tax, and in many cases you could end up paying more tax overall than if you file a separate return for the child.