What Is The Reason For Delayed Speech In Child?

my 2 1/2 year old son...has a speech delay he was talking really well people were amazed, all the sudden he just answers yes to everything or ignores what I say to him says nothing back. He does at times try to talk but he struggles...he understands what I say....eats good..sleeps good..plays everything is the same except he has lost his speech. I am really worried because he has met all his mile stones ahead of time and his Pedi says evaluation was great. We are doing a white and red blood cell blood test but I am hoping I can maybe get someone else opinion to help with this situation I am searching for answers to get my son back to himself. Thanks amie