Malcolm cant imagine how that must feel for you! I really cant. But he is an adult now and needs no permission from anyone to see you. Maybe try locate him, reach out, make contact, arrange to meet ? You are his father that counts and even though you have no been able to be his life it is not because you did not want to.
I know someone who had a similar situ but from the perspective of the child. They located there father. This happened when the child grew to a man around 25 years of age. They are now rebuilding there relationship. This young man was / is the step son of my brother. Who took him in as his own when he was a baby.
They have met and are building a new relationship and my brother welcomes this and supports this. There is always hope!