If he stays within a 100km radius then I’d say he’s exempt from the the EU regs, so no digi card needed, but will come under GB domestic regs, but doesn’t drive for more than 4 hours in a day and stays within 50km radius of base, then no records are required, if records are required then you can use a logbook. On a day when going over 100km then EU regs apply for that day and you’ll need to comply with the weekly rest requirements.
Also exempt from the dcpc so long as driving isn’t his main duty