OP, I'm here in PA as well and I have to say that I agree with LdiJ. I think if you prove to a judge that it's in your son's best interest as well as for the betterment of your education, career, etc., you won't have much trouble getting the move granted. Jurisdiction will remain in PA, and the commute isn't an issue. If he's in Montgomery County/King of Prussia area, it will actually be an easier commute down to DE than in to the city.
One thing that hasn't been suggested yet, however, is that I would advise you to send your ex a certified letter notifying him of your intended move (prior to you actually moving). Do you have anything in your current orders specific to moving away? Most states require at least 30 days notice. So send the letter asap. He'll have the opportunity to fight the move and take it to court. From there, you'll (1) know whether or not he's bluffing and (2) get the move approved legally.