I was 19 when my parents split... I have 3 younger siblings that were 12,9, and 3... I can tell you that no matter how old or young you are the dynamics change in the family.
My younger sisters were both mad at the world. The attitudes started popping up all over the place with EVERYONE. There were long periods of time that they refused to speak to our dad or mom.
As one of the older sisters I made sure to be there with them. I took them shopping with me, I took them on my dates with my future hubby (He still teases me that I was the most expensive girl he ever dated... But worth every penny.:-) My sisters all were in "love" with my now hubby) We watched movies together. Picnics, we went to parks, etc...
Is there any way that you can have him spend time with you? He is not alone. He has you guys. Don't try to make him talk about the divorce... But every day stuff. Things that are not wrapped up in emotion. Have him over for dinner and a movie... Take him to the zoo with you. Let him connect with YOUR kids as an uncle would. Take him on trips... (He's an extra pair of eyes and hands to help with supervision and babysitting. :-)
My brother is now 14. He spent many a summer at my house in MI. (he lives in PA)He LOVES my 8, 5 and 2 yr old...
Do not treat him like a child. He is preadolescent. When he is around (If he spends the night lt him stay up later...) Let him have more lee way than you would with your kids.
Do not choose sides in the divorce. No mom/dad bashing. Help him see that YOUR relationship with him will always be as a big sister and YOU will always be there. I bet you that inside he is feeling abandoned. But being a young boy can not or will not verbalise it.
Good luck!!!