Help!! my three year old son can't recognize or tell the difference between the letters 'b,d,p and q'.

his teacher at preschool had a talk with me and said that everyone else gets it but him and she's surprised since he's normally quite bright.

I've been spending the past three days trying to help him.. but no matter how many times I try he still doesn't get it.. I feel bad for forcing him cuz he's only three..but on the other hand I don't want him to fall behind. any tips on how I can make it fun and easy for him?