I can hear the pain coming through your words! It is very good that you arranged for your son to be evaluated and that you will be meeting soon with the staff. Those who evaluated your son should have a lot of information for you--what the tests indicate, what the school staff will do, and what you can do to help him. Ask lots of questions at your meeting(s) until you feel confident that you understand what is being shared and that you agree with the plan to help your son.
Whenever language difficulty is identified is the right time to start providing therapy and assistance--whether at 18 months, 24 months, 2 1/2, or 10. What's important is for everyone involved to roll up their sleeves and get going with a shared plan to help your son improve his language skills.
Aileen named many of the areas that are looked at in higher-level language, and there are many more. Press the staff for specifics regarding your son.
Best wishes.
Mary Lou B. Johnson, M.S.,CCC-SLP
http://www.helpyourchildspeak.com