grandson's phonics class says 'buh' not 'bee'. i've been teaching him wrong, apparently

he does a weekend phonics class. i pick him up most afternoons and we read signboards in the car, the way i learnt. a for apple, b, c. yesterday he told me, quite firmly, that b says buh and i was saying its name, not its sound. the teacher had told him grown-ups get it wrong. so now i'm learning sounds at 57. do the two ways confuse kids at this age, or does it sort itself out. should i just stop doing letters in the car.