Or he could just keep a gun on him, which easily fits on the hip, and as long as he is properly trained, it's more than enough protection, as this situation has shown. The border agent gets to go home safely to his family, the stupid shits that assaulted him didn't.
Also, riot shields are unwieldy for a bicycle, and are unnecessarily burdensome for someone that is supposed to be agile and lightly packed, i.e. a border patrol agent on a bike, which is the reason they are on a bike in the first place.