My son has been sleeping in his crib since he was 3 months old. We never let him sleep with us until recently because he was sick; now he won't go back to his crib. He throws a temper tantrum when we put him in the crib. He even started slamming his head against the crib, leaving black and blues on his forehead. Sometime he will drop to his knees, sit on them, and fall in-and-out of sleep for hours. He never lays down or falls fully asleep.
It sounds like you have one strong-minded little guy. Take a few days and play games in and around his crib to associate it with fun. Play peek-a-boo through the slats. Hide a favorite toy in his crib and go hunting for it. Do this for a few minutes several times a day. The next step can be putting him in the crib at bedtime and sitting with him, until he falls asleep, to avoid a total tantrum. Every few days, you can sit farther from his crib. Once he starts sleeping in his crib again, be consistent letting him sleep in it. Even if he gets sick again, continue having him sleep in the crib. You can sleep on an air mattress by his crib so you're nearby. It will be much easier to transition you out of his room than transition him back to the crib.