Struggling to support my 20+ year old son

My son is now in his mid-twenties, and his mother divorced me when he was 10 - and it was a very acrimonious divorce driven by her and her family.  

Despite my best efforts both before and after the divorce, and offering to help him in any practical way possible, he's not responded to any offers of help - other than purely financial (getting him out of debt, buying various items to help him live independently from his mother).  I also offered to pay for professional counselling about 18 months ago, which he's not responded to - either on his own, or with me if he wanted to.

He's said (and written) some incredibly hurtful things - and hasn't explained what he means or, for that matter, what influences (other people, social media, etc.) have made him reach those conclusions.  

After a gap of about 6 weeks or so (since the messages) he 'phoned me out of the blue recently and after a very rote conversation - and as soon as I mentioned the previous messages he told me to f**k off and I had to put the 'phone down.

Would be grateful for any suggestions of where to go from here.

Thank you.