Just let him know, over and over as quietly as you can that you are there for him if, or when, he wants to talk. Just stay his friend, that is the most important thing you can do for him right now. Maybe, in a casual moment, give him a "I'm a big brother" shirt, and a onesie to give the little sibling with "I'm the little brother/sister." Remind him that the child is innocent in all this, as is he, and regardless of how he feels about his mother right now, he IS a big brother.

(An aside: my mother had a baby at age almost 43 - you at Pet Talk all know him as Lady's Human, but she had me when she was 38, so it wasn't as big a gap!)

Then just remind Matt that he is an important person in your world, his having a new sibling doesn't change that in the least.

At some point in his life, of course, he is going to have to deal with the issues he still has with his parents' divorce. Better now than later, for everyone concerned.