Without much information given here in your question, my first thought is that if you believe your son may be having problems learning, you need to find out why. I would recommend that you first contact his teacher to get some insights into what is going on. If that does not help, then I would ask for him to be evaluated by the school psychologist. The sooner you get your son help, the better off he will be.
I went through the same with my daughter. Please dont make the mistake that I made and accept a "no" from the teacher. Your request to have him evaluated for a learning disability is your right. If the teacher says or believes he just "not trying hard enough" or whatever else, write a letter to the school principal and make a formal request that way.
It wasn't until I pushed and demanded it that way, that my daughter was able to be evaluated. In addition to that, I had her pediatrician write a prescription (which was more of a request to the school) to have the psychologist evaluate her.