M.R.
Have you noticed any issue with langague or speech, or is any other area of congative development a concern? The colors and shapes may be considered an issue at this point, but letters and numbers will not attract the attenion of a school district until he is well into kindergarten and they can verify that he has had appropriate instruction. Generally, a learning disablity cannot be identified until the child does not learn something that is expected, and those for schools anyway, will be in specific areas like reading, writen expression, or math calcualtion. No school will even evaluate a preschooler for a learning disablity. They will evaluate if you suspect a speech or language delay (and this could be a word retreival issue, if he also has trouble with things like identifying objects or other langague oddities that you have never assoicated with this, or an articulation issue.) He could also have a cognative delay, but most schools will only want to evaluate if this is effecting his adaptive behavior (meaning, life skills, like toileting, eating, self care.) They may evaluate if you think it is attention related, which is technically (in a school situation) not considered a learning disablity, though it does effect learning.
If you are just worried, you might check your insurance benefits and see if you have coverage for speech and langague evaluations. Many policies cover evaluations even if therapy is not a covered benefit. The speech therapist will screen for IQ, and can tell you if he has issues with langague that could be causing this. Especially if you are not seeing other developmental things that make you wonder, like nothing socially or behaviorally, then I would see if this kind of evaluation does not give you some answers, or ease your mind.
Once he gets to kindergarten, he will be screened, and if he does not know what he needs to know, you will be contacted about what to do then too, or you should be. I would stay vigilent and make sure that he does not slip through anyones fingers if he continues to have issues.
M.