I think you're being unreasonable.
It's his parents! Of course they are going to be interested in knowing how much he is going to be making (I could understand you not wanting to tell them how much you making).
I think it's fine for them to ask especially if they helped pay his tuition (not sure if they did or not). I also think you have to take in consideration his age- I believe that you are both fairly young, right? It's not like he is graduating from college all that many years later than the average college student.