Knowledge is the most important aspect of this case. The state has to prove some level of knowledge. Often they rely on "should have known" it a case where it is obvious that these two 17 year olds stole the merchandise. So it becomes very important to find out what the evidence against your son has been gathered. Has he or the other two boys given statements to the investigating officers, were fingerprints or some other evidence gathered at the locations. You really must find out what the evidence is against him to be able to determine if there is a strong defense based on knowledge. As for his disability that may be a defense as well, but that would have to be discussed as well, to determine if it rises to the level of a legal defense.
Based on the information that you've provided, it appears that your son has been charged with 3 Class C felonies, which typically will carry a 3-6 year punishment range. However, the judge will have the option to run any sentence consecutively or concurrently, which could have a tremendous impact on the total amount of punishment your son may be facing. You and/or your son should contact a criminal defense attorney immediately.