I would recommend that you continue following up with your grandson's pediatrician and/or get second or third opinions from other licensed pediatricians, particularly those who specialize in this area. Ask as many questions as necessary of these doctors until you feel comfortable with the accuracy of the diagnosis and learn what you can do to help your grandson.
 
For general information about Asperger's Syndrome (AS) and/or mental retardation, here are two related info centers on Education.com with a variety of articles in each:
 
http://www.education.com/topic/aspergers-syndrome/
http://www.education.com/topic/mental-retardation-child/
 
I'm also including below a couple of other additional resources below you may find helpful.

A pediatrician can be a good start.  However, I know that public schools are fully able to help you, as well.  Please contact your grandson's school and ask for a meeting to determine the need for a psycho-educational evaluation. (Give your reasons in WRITING and save all forms and information in the school in a file. Date all papers.)   In essence, you are asking to determine your grandson's intellectual abilities and strength and weaknesses in a variety of learning areas, including socialization.  A full team of specialists in the schools should be able to help to determine if your grandson has Aperergers, Mental Retardation or another learning disorder that might be inhibiting his success in school.
 
There are many rights and regulations for this process.  Feel free to ask your school for the federal guide to parent/legal guardians rights. They should offer you a copy and a full explanation at the beginning of every meeting you have with them.  
 
Please note that I based this answer assuming your grandson is in public school and you reside in the United States or one of the territories of the U.S.