Your son may also have been going through emotional dilemmas associated with his age and stage of development where other attributes are considered more valued by his peers than getting great grades. He is also distancing himself from his brother by not being like him in school.
Given these observations, I would have a few heart-to-hearts with your boy and offer him enough empathy so that he understands you have not given up on him, that you still believe in him. Offer him private tutorial help to jump start his climb back. This would also be a good way to find out if he has been struggling with some basic academic concepts. Emphasize that this is one small piece of time in his life and that you know he will land on his feet again. Measure him by several of his good qualities that are still present while you are talking with him. Let him know that he will always be special, regardless of whatever academic success he achieves. Encouraging effort and not results is a good framework to operate in.
I know you'll abide with your son; this is one of those bumps that tests families. You'll pass with flying colors.