Hello, my deepest condolences on your loss of Matty.
I am sure that you are in shock and grieving.
If he was home with her when she died or was dying it is possible that he recognized her passing as final and has accepted that she isn't coming back. As long as he knows that he likely will not be as lost as if she had disappeared without his being aware of her passing. Animals seem to accept Death better if they have knowledge of it.
When animals are very close I will often recommend that they be allowed to see a housemate's body after death so that they do have some understanding that their friend isn't coming back. I believe it allows them to heal and move on emotionally.
Animals live in the here and now. That isn't to say he doesn't miss her but he is aware that she isn't alive anymore, so he is aware there is nothing he can do to bring her back. This allows him to move on.
Please take care and be kind to yourself. I am so sorry your lost your pup at such a young age. Make sure to spend time with your fellow. He needs you now more than ever as an owner and a playmate now that he doesn't have Matty.