Like Grinity said, it's maybe it's because he hasn't had the best teachers in school or the most supportive parent at home (okay grinity didn't say that part, hehe).
LotsOtots5-
I most certainly didn't say that your haven't been the most supportive parent at home!!! You've provided 4 siblings and character education - can't school be responsible for academics? It works that way for 97% of the population, it can't be your fault for not knowing that you weren't in the usual situation.
Now that I see the fluency score at the mean and the GAI at 148 it all makes total sense ( at least to me ) - Your child is PG and 2E, and these kids just don't 'present' like the typical 'teacher pleasing' optimally gifted kid. I think it's fine to leave the school situation as is for now, but do fill out that Davidson Application. My son has similar scores and range between his highs and lows, and I didn't 'get wise' until he was 8, so you aren't so very far behind me, plus your are here now, so it'll be easier to catch up.
I hope you apply and get accepted to Davidson, because it makes such a difference to see that there are kids out there who are 'like' your son, and that they are 'really really smart' even though they don't do well in regular schools.
If I haven't recommended 'Different Minds' by Deirdre V. Lovecky, Ph.D let me add that to the library list - excellent insights into 2e kids.
Love and More Love,
Grinity