I'm not a mum but have had type 1 for a long time, and have been carb counting since the start (although it wasn't called that when I was diagnosed).
As a general rule, insulin resistance tends be associated with obesity and/or the ageing process (it's more complicated than that of course but that's a basic principle). So unless your son is overweight, he's unlikely to develop insulin resistance.
However, if he eats too many calories ('too many' = eating more than using up), whether those calories come from carbs or fat or protein, he may become overweight and may develop insulin resistance. This is true for everyone of course, not just your son!
It sounds as if you are getting to grips with it and I wouldn't worry too much about upping insulin doses if you need to. The most important thing is to match the insulin to the carbs (just as his body would do automatically if it could).