My son inherited stock from his grandfather and has sold it at a lower value than what it was originally purchased for. How is that taxed?

If he inherited it you don't use the original cost.  You get a step up to the value on date of death.  You can deduct a loss on your tax return.  Did he get a 1099B for the sales?

You can enter a 1099B manually or try to import it.  If you import it be sure to check it over close and make sure the cost basis got imported and is right.  How to import your 1099

https://ttlc.intuit.com/community/entering-importing/help/how-do-i-import-my-1099/00/26254

Enter a 1099B  under

Federal Taxes Tab or Personal (Home & Business)

Wages & Income

Then scroll down to Investment Income,

Then Stocks, Mutual Funds, Bonds, Other - click Start or Update

And If you have a net loss for the year you can only deduct 3,000 (1,500 MFS) max on your tax return.  The rest you have to carryover to next year.  You have to report the carryover every year until it's used up.  You can't skip a year.