It sounds like he has had a workup done for his low sugars as an infant but it is time to repeat things, alot has changed in 8 years, there is another type of glycogen storage disease that looks alot like ketotic hypoglycemia and this is called GSD type 0, Bill Winters is an endocrinologist in Florida who spends all his time in this field, If your doctor is doing a workup then involve yourselves with an endocrinologist as well-let them collaborate, theoretically 70-79 is not defined as hypoglycemia and it is very possible to have a surge in sugar in the 200's right after a carb load but this better resolve by an hour or two at least. There could be a glucose processing problem, there are forms of this called MODY or monogenic diabetes, sometimes these kids can have abnormalities with insulin secretion rather than autoimmune diabetes and do well unless excess carb load. Other thoughts include an organic acidemia or other rare but important problems, this is tested for with urine and serum for organic acids.
Have him seen, the symptoms also may not be related to his glucose per se since it was indeed in the 70's and came up as ours would and (hopefully resolved normally as well).
I would work with a genetics specialist and or an endocrinologist, the two work on metabolic problems, just simply cause he did indeed have the history. Hope this guides you.
I also forgot to add that he is VERY skinny. His is 50.5 in tall and 54-55 lbs. He is not very active as far as little boys go. He has, in the past few months complained of "my body is so tired" and "my back hurts". He had a negative spinal x-ray and blood tests that came back normal. (kidney function and thyroid I believe). Where do I go from here??