If the activities are sufficiently regular, continuous, and substantial, it's a §162 trade or business with its income reportable on Sch C and taxable as ordinary income at marginal rate.  Otherwise, the income may be reportable on Sch D.

You mentioned that he hasn't had any transactions in 2019.  While that may be a factor to consider, your key focus should be on the level of activities he undertook during 2018.