I do think Alston spent some time at DeMatha but not recently.
I did see this tweet from a couple of hours ago, and it looks like he finally became eligible and started playing for Herndon.
Nick Eilerson @FairfaxNick 4h
Senior transfer Brandon Alston scored 14 in his debut for Herndon last night, a 76-64 win over Oakton. Nice boost heading into postseason