Wow. Just curious what he said. Did you know him well? I only asked profs I knew would say yes and say something good.
Is there any way you could go meet with him? If you give him your entire transcript, and have done well or improved steadily, and you have a good work record to show for what you've done since you've graduated, he might be able to give you a recommendation.If you can give him this, and sit down and speak frankly with him about why you asked him, how you felt you performed in the seminar, and your other strengths, he could change his mind. I would definitely be clear, however, that if he still feels like he can't give you a great rec, then go for someone else.