Hi, I have a similar little boy, he just turned 4. As others said, GDD is a descriptive term, which later changes to learning disability/autism or various combinations.
I dislike the 'delay' bit so much, since it implies our chikdren will 'catch up', so I just use learning disability if I'm explaining it.
My ds had genetic testing, nerve conduction studies, MRI, lumbal puncture - all came clear, no 'reason' for his 'delay'.
Around 50% of children with LD there is no known cause, but if there was no birth trauma/ infection/ accident, genetic condition is suspected - we are now part of Deciphering Developmental Disorders genetic study.
It's very difficult not to know what the future holds :(. I hope everything goes well for your boy, it's great he is getting statement.