Help with PIP appeal

Hi

I need help please. My 39 year old son has ADHD . He's still waiting two years later for his assessment..He is struggling and already in last few months he's had 2 welfare visits from the police.

He had a appointment with a psychiatrist in September who tried to push for his assessment.

I applied for PIP for him..He had a phone assessment and has just received his papers back saying he's been refused .

Is there anyone who can point mein the right way to appeal please .

It's a huge worry.

Thankyou

Denise