Buttonjugs I am both with those who feel only pity and sympathy for the abused 15 year old you were when you had these liaisons that led to the birth of your son and also, absolutely with GSM in the advice she gives. She is a solicitor, so knows whta she is talking about.
At the end of the day, does it matter who his father was, you are his mother and raised him, his father has had little to do with him.
His tragic descent into drug addiction is not your fault. You yourself were used and abused as a child and I suspect may not have had an easy life since, either when he was young, or now and sadly the problems that affect struggling families keep going through the generations.
You are right to disbelieve anything your son says about his parentage, inheritance or anything else. He was watching or listening to a tv programme that featured it, and thought it a good 'joke' to use it to wind you up.
Be kind to yourself.