Clearly he knows romance exists, and has helped others succeed at it, but he's so blunt as well as analytical about everything that he can't see it when it's directed his way, and he has no interest of actively pursuing it anyway.

Isabella should take a hint.

Maybe he likes Isabella but has a hard time explaining his feelings.

I think he's gay.

He isn't dumb or oblivious in the slightest he knows Isabella likes him. And he obviously knows how to handle it. So I think this answer would do nicely. Even through you could complain that he's oblivious when Isabella is all over him. I think I doubt that since he has shown in the past that he saw how much she was madly in love with him. In some episodes he even tries to get rid of the subject and only sees the task in hand or what he's working on. I think he does have a feelings for Isabella but is probably like me not wanting to address it. Because of age. And besides if he wasn't Disney sure'll have a hard time getting the fans not to bash them for it.

According to what I've seen of the new episode he really hasn't noticed that Isabella likes him at all. He even states that he began liking her in high school.