I had always heard that puberty was the most common time for children to be diagnosed with T1D.
I found the following that suggests the same.
Puberty and T1D onset
I suspect the endo in this case most typically sees kids after they were in ER with DKA, dehydration, and BG's so high they don't even register on the meter (>600 for most meters).
What's challenging is what does that same child test like the months or weeks before that point. The up and down readings that you have reported may be typical, but most don't have a T1D in the family and test as early as you did, or even recognize early symptoms.
I don't agree with how the doctor responded, although you say he did indicate your son is in the 'really early onset stages'. Problem is they don't teach doctors how to treat kids in that condition. They just learn how to treat 'full blown', high ketones/BG and in some cases in or near coma. A great doctor, especially knowing there is a T1D parent, would certainly encourage monitoring, and testing, and treat as necessary based on patterns.
Given what you have described, your son IS making insulin, but seems sporadic. If you were to start to administer insulin, how much and when ? What if his pancreas is sputtering and not very predictable. You may want to read the many topics on LADA, and honeymooning, which deals with this unpredictable phase.