Hay , I'll try to make this as short as possible but it's a complicated situation 
i was married 8 years ago for a very short time to a man who was Infertile, we had donor ivf to conceive a child (my ds). As he pressured me a lot that it was his only chance to have a child of his own . Shortly after birth my ex lost all interest in me and ds , he discussed with me his failure to bond with a donor born child and we divorced not long after.
He is Lebanese and after divorce he would often threaten he would get a Lebanese passport for ds without my consent and asked multiple times if he could take him to Beirut , obviously I said no. He also has land ownership in Syria and has fitness traveled between the two countries (again I discovered this after marriage, very secretive shady guy)
Shortly after my son turned 2 I moved from London to Scotland to be better supported by my family as my ex was not supporting us at all , after divorce completed he had a couple of visits with ds always with me present , and never showed any interest in any official visitation or further bonding ,
I was happy to cut ties and move on I have since married and had two more children , we are a happy stable family and my ds has a great sense of identity within our family unit . He understands his donor heritage but he struggles to place my ex Into his story as he doesn't know or understand how he fits in.
He's 8 so it's simple terms when I explain , " mummy couldn't have a baby so the dr asked me what I wished for and I said a beautiful baby boy, he told me if I wished hard he could give me a special seed that might grow in my belly if I promised to love my baby boy forever " ( that's the story I tell him for now )
So this random guy who has no relationship with him or genetic tie is a step he just can't understand .
A Couple months ago I got a call from my ex demanding contact with ds , talking about his parental rights and how if I refused he would go to a solicitor to gain rights. He said he is now with a new woman who has two children and he wants her to meet ....wait for it ...... his son 
My instinct tells me as he is now in his 40s he hasn't told her he is Infertile as he didn't tell me before marriage either , and he is using ds as some sort of advantage to impress his new family.
So my question, Iv lived in Scotland for 5 years he has been in London with very little contact , (three separate , hour long visits in four years ). If he decides now to go for visitation would he have rights , technically he is on his birth certificate as his father , a married couple becomes responsible for any child born from chosen donor ivf while they are married .
My son doesn't know him at all , and gets very upset when I suggest he might get to know this person . He still has his surname from his birth registration but is known by our family name at school Dr etc etc .
Why steps do I take , do I need to officially register as his custodial , do I take him to court first before he has a chance ? I'm out of my depth and Iv tried so hard to ensure my sons sense of identity and upbringing is safe and inclusive as I'm sure the donor heritage will be hard for him later on.
Any advice welcome
Thank
You