My 6 year old write letters and numbers backwards?

My little boy just finished kinder. He has been going to school full time since he was three. I would think after 3 years of learning his letter and numbers he'd have mastered them by now. He gets almost perfect grades in everything but letters and number recognition. Sometimes he gets them right, but sometimes he would write all the numbers and letters backwards. Is this normal? should I be concerned? His father has a learning disability, not sure if he was diagnosed dyslexia but even at age 29 he reads at a 3rd grade level. I don't want my little one to not get the help he needs.