P.M.
I wish we had more information – I have more questions than answers. Has it been more than a day of this behavior? Have you asked your little guy why he's afraid to pee? It sounds like he's in pain. The catheter may have scraped a raw spot in his urethra. At any rate, I would definitely call the doc's office first thing tomorrow and tell them about this problem.
I'm assuming that when you describe the procedure as traumatic, your son experienced pain when it was performed. I've been catheterized, and it hurt. So your son could just be terrified and, as you say, traumatized. He could also be picking up on your sense of upset or outrage over the procedure, and that would probably intensify the whole experience for him. It's important that you be as calm, confident and comforting as possible while your son deals with this. If you are upset, he'll probably pick up on that, in addition to his own distress.
So sorry to hear this has been so difficult.