If the OP's son indeed has Type 1 (and I suspect he does with a several-hours postprandial BG of 150 [6-hour postprandial number was listed as 160 in another post, if memory serves]), I believe he is not eligible for testing through TrialNet. My understanding is that TrialNet tests only the biological siblings, half-siblings, and parents of children diagnosed with Type 1, but only up to a certain age (I was 55 years old when my then 11-year-old daughter was diagnosed with T1D, her father was 63 years old, and her half-sister was 24 years old; all three of us missed the age cutoff for TrialNet participation).
I was surprised to learn the following at my daughter's most recent endo visit in late June. When I asked which Type 1 autoimmune antibodies she tested positive for and what her C-peptide level was at diagnosis, I was told that "when the diagnosis is obvious" antibody and C-peptide testing is not always performed. Although a little disappointed, I can understand this. Those are expensive tests that I'm sure health insurance wouldn't cover unless requested and absolutely effing necessary. And I have absolutely no doubt that my daughter has Type 1 diabetes.
Sam is correct; the OP's son is technically not in the honeymoon phase as it is medically defined as the period of time when some of a PWD's beta cells resume production of some endogenous insulin after they have recovered due to the administration of insulin. A person with T1D does not "test negative" when they are honeymooning.
However, I would insist on a BG level (either fasting or not) to determine whether the OP's son has an elevated plasma glucose level.