Getting a diagnosis

I'm pretty sure that my 18 yr old son is on the autistic spectrum, but he doesn't want to discuss this or try to seek help at all. I have tried to address this but I think he doesn't want to be seen as 'different'. He really needs some help and support in order to cope with his anxiety as do we his parents. 

Is there anyone who has come across this before and has any advice?

Many thanks!