The child is an adult ... And has the right to decide who is informed of their life events. Just tell the "sperm donor" that.
How do you tactfully deal with a self-righteous absentee father who demands to know your child's college graduation date, but your child has told you not to breathe a single word to him about it?
If the father was intelligent, it would take virtually no time at all to Google that information and get the answer. However, because he's such a blow-hard, he probably wouldn't think of it.
In that case, you could say, "Bob, thanks for getting in touch with me about Dan's graduation. It's so nice that you want to share this special day with him. However, the last date I knew about was May 13th (I am suggesting that you make up a date that is not close to the actual date).
Since he is an absentee father, he probably won't remember to come. If he does show up at the school and no one is there and wishes to throw a hissy fit - you can just remind him that the date you gave him was tentative and he should have checked with the college just like you did.