While I'm fine with paying my own kid's education I really don't want to also pay for my stepkid to get a good education.
Is there a particular reason you don't want to pay for your stepkid's education?
It's expensive, and I don't even care if he drops out of college.
You're right college is expensive, but it's wrong to treat your step kid so differently than your biological child. The inequalities will be noticed by the kids, and it will cause hurt and resentment. It's best if you can find a way to help them both with the cost.
I don't want to pay for it.
You may not want to pay for it, but you decided to bring a step-child into your life and now you are expected to treat him like one of your own. Does one child deserve more love than the other just because one did not come from you?