I see where you are trapped here - you need professors to write two of your three letters, but you have been out of school for so long that this professor cannot honestly write you a good letter.
As someone else said, see if you can sit down and talk about your goals with him. Other things you can do: send him a summary of your work in class to jog his memory, your resume, and a draft of your SOP (but don't necessarily ask for his feedback on this, this is for his information only). Try to give him a handle of who you are and what led you to applying to graduate school at this time. At worst, he will only write the useless letter. At best, your information will trigger something specific about you in his memory, or he can talk about his new impressions of you. It still won't be a great letter, but it will be better than he could have done. I'm sure he feels bad that he can't give you a better one.