6 years on can i get another assessment for 25 ye old son

I obviously don't know the details of your and your son's situation. The thing about a diagnosis is that it's just a starting point. It's likely that autistic people will struggle. I guess that's well recognised. The autism diagnosis ought to help in identifying an appropriate approach. For example therapy should be tailored to his needs as an autistic person. Perhaps he could access therapy to help him navigate his problems.

Of course, that's not the question you asked, so I'm sorry if I've given inappropriate or unwelcome advice. I wish you both well in finding a way through this.

What was the original diagnosis and why do you think an update is needed?

This will give us a better understanding of the context so we can give more meaningful advice.

To answer the question however, if you want it for free then you need to get your GP to refer him for an evaluation, but if he is already diagnosed with autism and there is no strong evidence to suggest it is otherwise then your GP is likely to refuse. Wait times are running up to 4-5 years at the moment.

You could go private in which case the wait time in probably between several months and a year depending on the company.

As for your sons struggles, I would suggest he get setup with an account here and asks for himself for advice - it is all anonymous so he can give as much or little info as he wants when asking for advice. Getting him to do it himself may give him a sense of control and self advocacy which is likely to be positive for him.