My 13 year old son was diagnosed five years ago, and his specialist tells me that his body could be rejecting the insulin Could you please tell me why?

I am sorry to be cynical but this is very unlikely. The problem your specialist is referring to is, I think, production by the body of antibodies to insulin. Theoretically, these could attach to the insulin molecules and prevent them from working properly. While this used to be a problem on occasion with animal insulin, it is not an issue now that almost all patients use human insulin. While antibodies are produced, they are not at a level enough to interfere with function.

Far more likely is that your son is: (a) fed up with diabetes and not sticking to his diet and insulin plan as well as he used to, and/or (b) possibly skipping insulin injections (we know that this is extremely common), and/or (c) has lipohypertrophy at his injection sites from prolonged injection into the same spot which will interfere with insulin absorption. I leave it to you to appraise which you think is the most likely.

It is also worth noting that at 13, he is probably well into puberty, and his insulin requirement will be much more than previously. This is normal and should be accommodated by increasing his doses according to his blood sugars.

KJR