12 years old boy autism

Hi, my son is 12 years old. He is a very talented musician and does lots of work in various places of music professionally. He is studying in a secondary grammar school which is a mainstream school. But in different areas he needs support. He can't follow multiple instructions at a time. He can't understand people's emotions and doesn't know to show proper emotions at proper time. People think he is very mature and older because of his personality. If we don't teach him something in day to day life he can't do it. He can't jump and can't run properly till now. We had to teach him how to run. We had to teach him how to climb stairs. Before talking to someone we need to tell him what to say. But if he gets his favourite topic he will continue talking about it for hours. If he finds something interesting he goes for it and finds each and every single thing about it and it continues until I fight to stop it somehow and that takes ages to divert the interest etc. 

I am a LSA in a SEN school. Though since beginning I had doubt that he have some issues but as he is doing extremely good academically and in music, everyone said he is a genius. Now I think he has symptoms related to Asperger's syndrome. But as he is a very quiet well behaved and good student school is not helping me to get referral. GP is asking for school's assessment. 

Can anyone help me with this problem? As his problems are not quite seen normally by school or outside people, how can I get help to diagnose his problems?

Nabanita