my Dad (Lens Grinder for bomb sights) was in a 'reserved occupation'
In general, essential services for civilian life were exempt, and
essential services for the 'War Effort' were exempt.
Essential services would probably include most of those working in any factories that were producing goods and equipment needed for the fighting.
i.e. weapons, munitions, planes, ships and army vehicle manufacturers, military clothing manufacturers, parachute makers, etc
Part of those processes would need warehouse men, but unless we know more about what was in his warehouses, we can't really say.
Unless it was very specialised, then I'm with Ricky on this one: probably women took over his job and he would have had to join the forces.