Teen friendship problems

Sorry if this is covered in another thread, I tried to search but couldn't work out how. I'm happy to be directed to another relevant thead 

My 16 yr old son with autism is having problems with a 'friend'. The friend's mum says that my son is contacting him too often and the content of the emails and social media posts are concerning. I haven't seen them.

He's been in this situation a couple of times before and I have spoken to him. As far as I know he has backed off when told the other person is uncomfortable.

I don't know how to broach it with my son this time , any advice? They've known eachother for more than 10 years and have been good friends in the past.