Some horses do swish their tail when they're stressed. But more often it is when they're in pain. Since lunging causes all sorts or pressures/strains that aren't exactly natural, I'd be more inclined to link it to pain/discomfort rather than 'just' stress. (I say 'just' as pain/discomfort does cause a stress reaction in horses.) It could well also be the reason that he doesn't like to lunge.