My guess is that this is totally normal and it's too soon to really worry about anything yet. Number reversal is really common, even as they get a little older. My daughter is 4 and sometimes she writes out her name, but if she runs out of room on the paper, the rest of the letters will end up wherever she decides to put them. I think kids at that age don't realize that letters or numbers have to be in a certain order and if they are in a different order, it will have a totally different meaning. I was helping out in my daughter's preschool class the other day and one little boy wanted to write STOP on a piece of paper. When I told him the letters, he wrote it right to left, so he spelled POTS instead. I corrected him and reminded him that we write left to right. At this age, they cannot always remember which is which, so it's easy to get things mixed up.
If you can do a "junior kindergarten" class next year instead of full-on kindergarten, you might want to consider that. Hopefully he would get more out of it than just another year of preschool and will be better prepared for K the following year.