My 6yr old son is in 1st grade.He is an A student but has a hang-up w/the letters D&B writing them backwards upper & lower case,sometime. why and why these letters. He will also write his 7,5and 3 backwards. What could be behind this?

If he were 8 or 9 I'd be concerned, but this is very common for 1st and 2nd graders. You can confirm this with your son's teacher. Don't worry about it.