How to go beyond 60mg inspiral limit.

My body has a fast metabolism. I eat a lot and I don't gain much weight. I drink a lot and my pee is light yellow yet I still pee just 2-3 times a day. That's why I also need a higher dosage of meds than normal people. I'm on 60mg Ritalin SR. I take 30mg in the morning and 30mg more in the afternoon. One shot just lasts 3-4 hours for me(it normally lasts 6-8 hours for people) and ritalin IR doesn't even last 2 hours. And this is when I'm the highest approved dosage of ritalin which is 60mg. A single dosage lower than 30mg doesn't do anything for me. It lasts just 6-8 hours collectively on a good day. That's why I need I think I need a higher dosage, which I think is 30mg more in the afternoon so that the effect lasts 9-12 hours. This is way over the maximum dosage my doctor has agreed to prescribe me. What tests do I need to get done for me to convince him that I can handle a higher dosage? I'm 15 year old male and I'm 49kg.