You need to contact your local public school. You should be able to do this in person and request that your son be tested for a learning disability. I wish you well.
how do I write a letter to have my son tested for a learning disability?
Most of the time it done through the school .
Thats how it's done in the school In Ireland
The school or playgroups are usually very Quick to pick up on things like this.
You could Probaly contact your Health Service
Best of Luck