I don't think the father has authority over the child now. If the son is a full adult then I wouldn't suggest a father trying to dictate anything to the child, even if the father had been present in the child's life from birth. Since the father has been absent from the child's life for some time there is a reasonable chance the son could resent or be angry at a father for trying to act as if he has authority now.
As to how the relationship between the father and son should work out, that is more complicated. Really in my mind this is up to the son to decide. The father should speak to the son and try to get a feel for what the son is looking for in his father, if the son missed not having a father and wants to know the experience then the father should allow their relationship to grow into something more like that of a traditional father-son relationship. If the son is mature and seems less interested in committing emotionally to the father, which is very possible, then the father should allow the relationship to evolve into one closer to mutual respect as equals. In general I would error on the side of treating a child as an adult and equal. Look at the way fathers that have been in a sons life since the son was a kid treat a grown son now; usually their relationship has many elements of treating each other as equals already.
In either case the father is going to have to be prepared to answer the big questions, such as why weren't you in my life and did your love my mother. If the father was just an anonymous sperm donor these questions may be easy; but if the father knew the mother this can potentially be an emotional minefield the father had better be ready to address. Even if the father was just a sperm donor the son may want to know why the father donated; or if the father ever thought about the children he was fathering.
