Struggling with diagnosis because of gp

How do I get a diagnosis for my 4 year old if my gp just doesn’t cooperate. 
I’ve been back and forth with my son to the GP but they don’t seem to be taking me seriously. 

Pre-school have been amazing they have referred him to sleep clinic, doing asset check with him. The behaviour team have been in and they have said it looks like he’s got PDA autism and have referred him to there team but the gp just don’t want to know