A 14 pound weight loss in three weeks is a cause for serious concern, regardless of the possible ADHD and possible learning disability. There is really no such thing as a 'borderline' learning disability. Either there is a disability or there is not. The challenge is to figure out precisely what is occurring, and the only way to know for sure is for your child to undergo some testing of his cognitive functioning. If you have not already done so, request an evaluation through the public school. That is always a reasonable way to begin, and often answers the pertinent questions. The ADHD question can be evaluated via a mental health evaluation. You can ask your son's pediatrician for a recommendation of a clinician who can do such an evaluation. Before anything else, though, arrange an asap appointment with his pediatrician - the weight loss is dramatic and worrisome.
When your son sees the pediatrician, you may want to ask specifically about having him tested for celiac disease. It is a gastrointestinal disease that can cause malabsorption and associated weight loss, but can also cause cognitive symptoms (due to lack of absorption of vitamin B12). I mention celiac disease in particular, because it is somewhat more common in people with type 1 diabetes than in the general population. Since you have a son with diabetes, possibly your other son, who of course shares much of the same genetic material, may also have an elevated risk of celiac disease. By the way, if any member of the family turns out to have celiac disease, it is a good idea to test the other members of the family as well, since celiac is an inherited disease.