Honestly, if frankness is your personal tendency, why deviate?
It's not as scary as you think. Whenever I meet someone that I would consider dating, I tell them ASAP that I'm in an open marriage. It's not fair to let someone develop feelings for you thinking you're single.
You've already shared some intimate moments. I'm not sure what you're so worried about. He has feelings, of course, everyone does. But it's not like you're telling him he's defective or something. This news is about YOU.
If you're still nervous, feel him out with the old "I have a friend" trick and see what he thinks about the idea of polyamory. Shit, if he's a senior in college, I'm sure he'll think that's great! You're offering to let him have his cake and eat it too.
But do it soon! The longer you wait, the more it's going to look like you were deliberately hiding it from him and being dishonest. And that won't be a good way to start a relationship...
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker