I think I would try to get DH to observe an academic situation, though sometimes a non-academic situation can work if he can get that the kids who are your child's age aren't little kids or backwards somewhow--they're the "normal" ones.
I'm the researcher in our family, too. Can you boil down for your DH the most important or shocking things that you find in your research? Maybe you're doing this already...
Have you explained to him that learning helps you to get a handle on the situation? Have you told him how out of control and helpless (or whatever) you feel in the face of the situation? Does he know what you fear? (Do you know what you fear? Naming that may be the first step.)
It sounds like he doesn't understand the seriousness of the situation and how it's affecting you and your son. If he thinks you're making a mountain out of a molehill, he's not going to be as supportive. You probably have to show him the mountain before you do anything else. Make sure he knows why easy As can be problematic--perfectionism, chronic underachievement, etc. I think communication is key.
I also think you're probably right that he may be experiencing some denial. It's not a problem if he doesn't admit that it is. Easy is good when it comes to schoolwork, right? So no problem...
What if DH does some of the homework with your son? Maybe on the weekend, if after work is too much? Or if he can't do that in the evenings, maybe contrast for your DH some afterschool work that you assign that is at your son's level with what he's being assigned in class? I think seeing the difference in front of him might help your DH.
I'm not sure that's helpful, but I'll keep thinking. Keep talking if it helps. Maybe we'll get closer to something useful if we're not there yet.
