Educators need time to view what is going on with a student who has problems.  Teachers begin to screen and evaluate. To make sure that they do a good job they would want to RTI a student and make certain that no  eye problem or hearing problem or other medical problem was evident before making a decision and that does take time.

When you say he mixes up his letters when they stand alone, is it the name of the letter he has difficulty recalling?  With the numbers, is it the name of the number he can't recall?  Can he show you the letter or number if you name it for him?  Some children can learn to read by identifying the letter by its sound and not its name.  But they have difficulty with letters alone because they are asked to "name" the letter.  Numbers do not have sounds and therefore it may be a "name" issue.  Its great that you have already pursued testing for your child!  That should give you the answers you are looking for and provide the help your child needs.

Yep, it's a name issue. Very frustrating. I'll think he's finally got it only to ask him next week and he'll mix them up again. Sometimes he knows them and sometimes he doesn't. There are other things too. His fine motor is in the toilet. He also has some auditory processing issues I think,
though he did learn to talk early and well. We'll see what the evaluation says.