Hello, welcome!
From my understanding the gauge is the thickness of the needle and your son prefers a finer needle. If it's the same brand of needles but different gauges, would speaking to the pharmacist be of any help to swap? or speaking to the GP to specify it clearly. But check his prescriptions, some times its the same box of needles but the gauge varies between boxes. If you can also contact his consultant or DSN to say the GP has been prescribing the wrong item, maybe they will write a letter to state it clearly - GPs based prescriptions of the consultant's letter. I never had a gauge problem, but a needle length problem where I've been given 8mm instead of 4 or 5mm needles. CCGs were trying to save money by giving me cheaper needles and the wrong length.