I'm an occupational therapist and can tell you that this is within normal range for his age. In my field we call this skill "directionality" and it improves with age. If a child is doing this at 7 or 8 it would be a concern.
In the clinic, what I would do is start w/ body awareness (this is a precursor for directionality). I might give a child a sticker and have him put it on his head, his hand, his knee, his foot etc. Then we would progress to other types of directions such as "above, below, on, in, under, on top." I would make it a game. For example, you can get a chair and put a toy "on" it, "under" it, to the "right" of it. Get him to say where it is and take turns placing the toy.
Directionality is related to reading and writing b/c you read & write from left to right, top to bottom. It takes time for kids to understand this concept.
If he's written his name hundreds of times correctly, then I wouldn't be concerned about him doing it this way a few times (or in your case, once).
It doesn't sound to me like your son has any sort of problem. A lot of parents will see this and think their child is dyslexic. In fact, most children who write letters flipped backwards, mirror image, upside down, etc. are actually still learning directionality....