I have never been in a similar situation so it is hard to offer advice also, I imagine he is feeling really left out and that he won't matter anymore, which is understandable, but probably not the case, just try and get him to open up if you can, if he can talk about it I am sure it will make him feel a lot better, to express his concerns and feelings, however he really should be doing that to his mother and maybe father as well, but if he does confide in you, just be there for him.

I hope in time he will grow to love his new half sibling, sibling rivarly is sad for him and the newborn, hopefully in time he will see his mother's love is still there and always will be, he will no longer feel threatened and may even grow to love his little brother or sister.

Let us know how you get on, just try and be patient with him. GOOD LUCK.