When he is batting? Looks like an old bottle opener, but I know that can’t be what it is
Generic term is a thumb guard, amazingly enough. It was developed to protect players against the sting of an inside pitch that jams them on a wood bat, but some players think it affords them other benefits. This article explains it pretty well.
<LINK_TEXT text=“https://www.azcentral.com/story/claytho … /10216697/”>https://www.azcentral.com/story/claythompson/2014/06/09/the-mysterious-baseball-device-that-seems-to-be-a-hit/10216697/</LINK_TEXT>