Okay, so I think Adam is definitely a LI you have to crack, in the way that you have to have him at high love and trigger a certain event for him to even consider you as his s/o... It's like how Robin has certain conditions that must be met before you can romance them, but Adam's mechanics obviously don't require you to take on a debt or some shit...

Adam can be unlocked as a LI if you have him at full love, full approval, full dominance, and at full lust. And I think once that's achieved, you will trigger events in certain places wherein it's clear that he wants you, and you can either choose to reciprocate, deflect, or to outright reject him then and there. Rejecting him doesn't permanently block him from being an LI for the run, same as how you can choose to reject Sydney and change your mind later. The difference is that these events just don't trigger randomly anymore, and instead, you have to approach him during specific times (weekdays at Fishing Rock specifically) to tell him you've changed your mind.

With my ramble about his mechanics aside, his main thought beneath being ecstatic that his loser ass finally has a girlfriend is guilt, because he's essentially cucking his dad with a person he views as his younger sibling that he just decided that he needed to protect. And to add onto that, Eden won't even be aware that you're dating him because his selfish ass definitely wouldn't share you, least of all with his own son. So he's guilty AND scared as shit.

And now you may ask: "But David, doesn't this go against the old dadson lore you had planned?" and to that, I say: Yes! I find the idea of him cucking his own dad hotter, sue me. I briefly thought of PC bringing up how he's better than his dad and he tells you to shut up but you notice how his thrusts get quicker and that made my weewee hard, therefore, it's the new lore.