I'm probably going to flamed for this, but here's my opinion:
Firstly, congrats to the OP for almost being done with your own schooling!
I honestly don't think you're under any obligation to pay for your SS's schooling. There are many universities out there where you can get a quality education for much less than $17,000 a year. To claim he didn't know how much the school was until after he had been accepted shows a lack of planning and responsibility. Honestly, that should have been one of the first things he looked in to when he considered this school a top contender. That also should have been something discussed long before he actually applied for the school - especially if the original plan was to attend a community college first (a cheaper way to get Gen Ed classes out of the way).
I think the way he handled it ("It's better to ask for forgiveness than for permission" kind of attitude) is a less than adult way to handle this situation.
Will he be commuting, or is this living on campus? What will he do for book money? Food? And obviously he'll want to keep up some social aspect of his live, where will the money come from for those things? What about bills? Sounds like someone needs a job.
Speaking as a broke college student with several jobs, my parents only foot any bills when an emergency or unplanned event happens (like I owed taxes because a camp I worked last summer didn't take any out and filed me as self-employed) but they do not chip in for tuition, normal bills (cc, car insurance, cell phone), my car (paid in full by myself), books, etc. In fact, because they offer me a rent free place to live (I commute the 45 min. to school) I contribute to the house when I can (groceries, I allow the family to share my Netflix plan, etc.). And it's taught me responsibility with my money.
If you still feel the need to pay the $3k your SS has surprised you with, I agree with other PPs. Explain to him that you currently do not have the finances to help him due to your schooling (which, being that close to being done should not have to be put on hold) and have him take out a loan. IF he graduates, pay that loan off. IF he decides to drop out, or he fails out, let him know that the loan will then be his responsibility. And, as another PP said, whatever you do for SS, be prepared to do the same for your other children.
Now, good luck with your final year of Pharmacy school!
