Internet help

Hi. I am a parent of a 18 year old autistic boy and I need some help. He is developmentally about 13 years old. 

He has had access to the Internet and I have found in the search history pages relating to wegies and porn. I have had the discussion with him about sex and pornography but he keeps going back to the wegies videos and the search is starting to go towards girls rather than women.

How do I get him to stop? Where can I get help? 

I'm not very tech clever but willing to learn, if someone can point me in the right direction. 

I am also looking for maybe a counselling service or help group who can give me some advice on how to deal with allowing him Internet access in a safe and responsible way.

Hoping for some guidance and advice. 

Poppins