Here are a few to start:
1. ChairScholars Foundation (http://www.chairscholars.org/
)--National Scholarships up to $20,000 for students with disabilities. Due in February 2008.
2. Horatio Alger National Scholarship (www.horatioalger.org
)--National Scholarships up to $20,000 for students who have shown perseverance in overcoming adversity. Due mid-October 2007.
3. Dell Scholars Program (www.dellscholars.org
)--National Scholarship up to $20,000 for students who have overcome adversity and have participated in an approved college-prep program (GEAR UP, UPWARD BOUND, etc.). Due January 15, 2008.
Another option is to do a word-specific google search for scholarship opportunities. A quick search resulted in this website: http://www.finaid.org/otheraid/ld.phtml
I am a 2006 high school graduate with a physical disability, and I applied for over 75 scholarships throughout high school. I ended up winning 18 scholarships--8 of which were national. Most scholarships are awarded for more than just academic records. The way I look at a scholarship (and as most scholarship organizations look at students) is that it's a financial investment in a student's potential--how well will this student do in college, how well will they represent what the organization stands for, how likely are they to give back. Thus, besides academic records, they look at your extracurricular involvement, leadership, and community service. If you have those, combined with an excellent academic record and strong personal story, your son would make for a competitive scholarship applicant.
If you have any questions, feel free to send me a PM. I'd be happy to answer them.