M.R.
Not to worry at age 5! If you think about it, he has just entered the learning phase where order and orientation matters. He just does not get that concept yet, and it is developmentally on target. Think about it, a chair is a chair, if it is upside down, but a b is a q is a d is a p...but they are all the same shape, and number order is the same right now. 87 and 78 is the cone on top of the ice cream or on the bottom? It is still an ice cream cone...
If he has issues, you will see them once he starts to learn concepts and learn to read. The issues you need to be concerned about specifically for dyslexia are the inabability to manilpulate and identify phonemes (the smallest sounds) and a problem with the symbols that represent them. If you see that problem as he begins to learn reading, have him evaluated sooner, rather than later and advocate that he receive an Orton Gillingham or alphabet phonics dyslexia instructional program to learn to read. The sooner kids with reading issues get this the better.
Visual processing issues can cause this kind of trouble, but you would be seeing a whole slew of other issues with him, and handwriting would most likely be a huge struggle. Occupational therapists and Developmental Optomistrists can assess visual processing issues.
At 5, I would not think anything of it unless he continues to struggle and does not improve as he moves through K and 1st grade.
M.