insulin is a necessary hormone for normal metabolism. it is also a growth hormone.
if his a1c is great then I’d have to guess that his daily blood sugars are predictable and relatively speaking. in “control”.
then you say he needs more and more insulin, which then says, I am guessing, his sugars are starting to run higher…and this is the reason for needing more insulin.
if you eat a lot of carbs and take a lot of insulin you will get larger. exercise may be able to balance the difference and would then be the only thing keeping him from getting larger. just like if he didn’t have diabetes, less calories in general and less carbs in particular, should reduce his need for insulin and then , in conjunction with his walking, should result in a loss of weight.
BMI is no mystery, you can go to any of a number of web sites and put in your height and weight and compare your BMI with ideal BMI, but it kind of generalizes and truly doesn’t tell the whole story.
if he is reducing his carbs and maintaining his walking, and his blood sugar is still climbing and he needs additional insulin… then it is possible he may be highly insulin resistant and may benefit from going to an endo and perhaps if the endo thinks it is appropriate, can add “type 2” medication to help him absorb the available insulin. Insulin resistance can increase with infection, with steroids, with stress, or by metabolic disorders such as type 2. I urge you to check in with the endo and I wish you good luck.